/usr/share/pyshared/storm/store.py is in python-storm 0.19-1.
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# Copyright (c) 2006, 2007 Canonical
#
# Written by Gustavo Niemeyer <gustavo@niemeyer.net>
#
# This file is part of Storm Object Relational Mapper.
#
# Storm is free software; you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License as
# published by the Free Software Foundation; either version 2.1 of
# the License, or (at your option) any later version.
#
# Storm is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
#
"""The Store interface to a database.
This module contains the highest-level ORM interface in Storm.
"""
from copy import copy
from weakref import WeakValueDictionary
from operator import itemgetter
from storm.info import get_cls_info, get_obj_info, set_obj_info
from storm.variables import Variable, LazyValue
from storm.expr import (
Expr, Select, Insert, Update, Delete, Column, Count, Max, Min,
Avg, Sum, Eq, And, Asc, Desc, compile_python, compare_columns, SQLRaw,
Union, Except, Intersect, Alias, SetExpr)
from storm.exceptions import (
WrongStoreError, NotFlushedError, OrderLoopError, UnorderedError,
NotOneError, FeatureError, CompileError, LostObjectError, ClassInfoError)
from storm import Undef
from storm.cache import Cache
from storm.event import EventSystem
__all__ = ["Store", "AutoReload", "EmptyResultSet"]
PENDING_ADD = 1
PENDING_REMOVE = 2
class Store(object):
"""The Storm Store.
This is the highest-level interface to a database. It manages
transactions with L{commit} and L{rollback}, caching, high-level
querying with L{find}, and more.
Note that Store objects are not threadsafe. You should create one
Store per thread in your application, passing them the same
backend L{Database<storm.store.Database>} object.
"""
_result_set_factory = None
def __init__(self, database, cache=None):
"""
@param database: The L{storm.database.Database} instance to use.
@param cache: The cache to use. Defaults to a L{Cache} instance.
"""
self._database = database
self._event = EventSystem(self)
self._connection = database.connect(self._event)
self._alive = WeakValueDictionary()
self._dirty = {}
self._order = {} # (info, info) = count
if cache is None:
self._cache = Cache()
else:
self._cache = cache
self._implicit_flush_block_count = 0
self._sequence = 0 # Advisory ordering.
def get_database(self):
"""Return this Store's Database object."""
return self._database
@staticmethod
def of(obj):
"""Get the Store that the object is associated with.
If the given object has not yet been associated with a store,
return None.
"""
try:
return get_obj_info(obj).get("store")
except (AttributeError, ClassInfoError):
return None
def execute(self, statement, params=None, noresult=False):
"""Execute a basic query.
This is just like L{storm.database.Database.execute}, except
that a flush is performed first.
"""
if self._implicit_flush_block_count == 0:
self.flush()
return self._connection.execute(statement, params, noresult)
def close(self):
"""Close the connection."""
self._connection.close()
def commit(self):
"""Commit all changes to the database.
This invalidates the cache, so all live objects will have data
reloaded next time they are touched.
"""
self.flush()
self.invalidate()
self._connection.commit()
def rollback(self):
"""Roll back all outstanding changes, reverting to database state."""
for obj_info in self._dirty:
pending = obj_info.pop("pending", None)
if pending is PENDING_ADD:
# Object never got in the cache, so being "in the store"
# has no actual meaning for it.
del obj_info["store"]
elif pending is PENDING_REMOVE:
# Object never got removed, so it's still in the cache,
# and thus should continue to resolve from now on.
self._enable_lazy_resolving(obj_info)
self._dirty.clear()
self.invalidate()
self._connection.rollback()
def get(self, cls, key):
"""Get object of type cls with the given primary key from the database.
If the object is alive the database won't be touched.
@param cls: Class of the object to be retrieved.
@param key: Primary key of object. May be a tuple for composed keys.
@return: The object found with the given primary key, or None
if no object is found.
"""
if self._implicit_flush_block_count == 0:
self.flush()
if type(key) != tuple:
key = (key,)
cls_info = get_cls_info(cls)
assert len(key) == len(cls_info.primary_key)
primary_vars = []
for column, variable in zip(cls_info.primary_key, key):
if not isinstance(variable, Variable):
variable = column.variable_factory(value=variable)
primary_vars.append(variable)
primary_values = tuple(var.get(to_db=True) for var in primary_vars)
obj_info = self._alive.get((cls_info.cls, primary_values))
if obj_info is not None and not obj_info.get("invalidated"):
return self._get_object(obj_info)
where = compare_columns(cls_info.primary_key, primary_vars)
select = Select(cls_info.columns, where,
default_tables=cls_info.table, limit=1)
result = self._connection.execute(select)
values = result.get_one()
if values is None:
return None
return self._load_object(cls_info, result, values)
def find(self, cls_spec, *args, **kwargs):
"""Perform a query.
Some examples::
store.find(Person, Person.name == u"Joe") --> all Persons named Joe
store.find(Person, name=u"Joe") --> same
store.find((Company, Person), Person.company_id == Company.id) -->
iterator of tuples of Company and Person instances which are
associated via the company_id -> Company relation.
@param cls_spec: The class or tuple of classes whose
associated tables will be queried.
@param args: Instances of L{Expr}.
@param kwargs: Mapping of simple column names to values or
expressions to query for.
@return: A L{ResultSet} of instances C{cls_spec}. If C{cls_spec}
was a tuple, then an iterator of tuples of such instances.
"""
if self._implicit_flush_block_count == 0:
self.flush()
find_spec = FindSpec(cls_spec)
where = get_where_for_args(args, kwargs, find_spec.default_cls)
return self._result_set_factory(self, find_spec, where)
def using(self, *tables):
"""Specify tables to use explicitly.
The L{find} method generally does a good job at figuring out
the tables to query by itself, but in some cases it's useful
to specify them explicitly.
This is most often necessary when an explicit SQL join is
required. An example follows::
join = LeftJoin(Person, Person.id == Company.person_id)
print list(store.using(Company, join).find((Company, Person)))
The previous code snippet will produce an SQL statement
somewhat similar to this, depending on your backend::
SELECT company.id, employee.company_id, employee.id
FROM company
LEFT JOIN employee ON employee.company_id = company.id;
@return: A L{TableSet}, which has a C{find} method similar to
L{Store.find}.
"""
return self._table_set(self, tables)
def add(self, obj):
"""Add the given object to the store.
The object will be inserted into the database if it has not
yet been added.
The C{added} event will be fired on the object info's event system.
"""
self._event.emit("register-transaction")
obj_info = get_obj_info(obj)
store = obj_info.get("store")
if store is not None and store is not self:
raise WrongStoreError("%s is part of another store" % repr(obj))
pending = obj_info.get("pending")
if pending is PENDING_ADD:
pass
elif pending is PENDING_REMOVE:
del obj_info["pending"]
self._enable_lazy_resolving(obj_info)
# obj_info.event.emit("added")
elif store is None:
obj_info["store"] = self
obj_info["pending"] = PENDING_ADD
self._set_dirty(obj_info)
self._enable_lazy_resolving(obj_info)
obj_info.event.emit("added")
return obj
def remove(self, obj):
"""Remove the given object from the store.
The associated row will be deleted from the database.
"""
self._event.emit("register-transaction")
obj_info = get_obj_info(obj)
if obj_info.get("store") is not self:
raise WrongStoreError("%s is not in this store" % repr(obj))
pending = obj_info.get("pending")
if pending is PENDING_REMOVE:
pass
elif pending is PENDING_ADD:
del obj_info["store"]
del obj_info["pending"]
self._set_clean(obj_info)
self._disable_lazy_resolving(obj_info)
obj_info.event.emit("removed")
else:
obj_info["pending"] = PENDING_REMOVE
self._set_dirty(obj_info)
self._disable_lazy_resolving(obj_info)
obj_info.event.emit("removed")
def reload(self, obj):
"""Reload the given object.
The object will immediately have all of its data reset from
the database. Any pending changes will be thrown away.
"""
obj_info = get_obj_info(obj)
cls_info = obj_info.cls_info
if obj_info.get("store") is not self:
raise WrongStoreError("%s is not in this store" % repr(obj))
if "primary_vars" not in obj_info:
raise NotFlushedError("Can't reload an object if it was "
"never flushed")
where = compare_columns(cls_info.primary_key, obj_info["primary_vars"])
select = Select(cls_info.columns, where,
default_tables=cls_info.table, limit=1)
result = self._connection.execute(select)
values = result.get_one()
self._set_values(obj_info, cls_info.columns, result, values,
replace_unknown_lazy=True)
self._set_clean(obj_info)
def autoreload(self, obj=None):
"""Set an object or all objects to be reloaded automatically on access.
When a database-backed attribute of one of the objects is
accessed, the object will be reloaded entirely from the database.
@param obj: If passed, only mark the given object for
autoreload. Otherwise, all cached objects will be marked for
autoreload.
"""
self._mark_autoreload(obj, False)
def invalidate(self, obj=None):
"""Set an object or all objects to be invalidated.
This prevents Storm from returning the cached object without
first verifying that the object is still available in the
database.
This should almost never be called by application code; it is
only necessary if it is possible that an object has
disappeared through some mechanism that Storm was unable to
detect, like direct SQL statements within the current
transaction that bypassed the ORM layer. The Store
automatically invalidates all cached objects on transaction
boundaries.
"""
if obj is None:
self._cache.clear()
else:
self._cache.remove(get_obj_info(obj))
self._mark_autoreload(obj, True)
def reset(self):
"""Reset this store, causing all future queries to return new objects.
Beware this method: it breaks the assumption that there will never be
two objects in memory which represent the same database object.
This is useful if you've got in-memory changes to an object that you
want to "throw out"; next time they're fetched the objects will be
recreated, so in-memory modifications will not be in effect for future
queries.
"""
for obj_info in self._iter_alive():
if "store" in obj_info:
del obj_info["store"]
self._alive.clear()
self._dirty.clear()
self._cache.clear()
# The following line is untested, but then, I can't really find a way
# to test it without whitebox.
self._order.clear()
def _mark_autoreload(self, obj=None, invalidate=False):
if obj is None:
obj_infos = self._iter_alive()
else:
obj_infos = (get_obj_info(obj),)
for obj_info in obj_infos:
cls_info = obj_info.cls_info
for column in cls_info.columns:
if id(column) not in cls_info.primary_key_idx:
obj_info.variables[column].set(AutoReload)
if invalidate:
# Marking an object with 'invalidated' means that we're
# not sure if the object is actually in the database
# anymore, so before the object is returned from the cache
# (e.g. by a get()), the database should be queried to see
# if the object's still there.
obj_info["invalidated"] = True
# We want to make sure we've marked all objects as invalidated and set
# up their autoreloads before calling the invalidated hook on *any* of
# them, because an invalidated hook might use other objects and we want
# to prevent invalidation ordering issues.
if invalidate:
for obj_info in obj_infos:
self._run_hook(obj_info, "__storm_invalidated__")
def add_flush_order(self, before, after):
"""Explicitly specify the order of flushing two objects.
When the next database flush occurs, the order of data
modification statements will be ensured.
@param before: The object to flush first.
@param after: The object to flush after C{before}.
"""
pair = (get_obj_info(before), get_obj_info(after))
try:
self._order[pair] += 1
except KeyError:
self._order[pair] = 1
def remove_flush_order(self, before, after):
"""Cancel an explicit flush order specified with L{add_flush_order}.
@param before: The C{before} object previously specified in a
call to L{add_flush_order}.
@param after: The C{after} object previously specified in a
call to L{add_flush_order}.
"""
pair = (get_obj_info(before), get_obj_info(after))
self._order[pair] -= 1
def flush(self):
"""Flush all dirty objects in cache to database.
This method will first call the __storm_pre_flush__ hook of all dirty
objects. If more objects become dirty as a result of executing code
in the hooks, the hook is also called on them. The hook is only
called once for each object.
It will then flush each dirty object to the database, that is,
execute the SQL code to insert/delete/update them. After each
object is flushed, the hook __storm_flushed__ is called on it,
and if changes are made to the object it will get back to the
dirty list, and be flushed again.
Note that Storm will flush objects for you automatically, so you'll
only need to call this method explicitly in very rare cases where
normal flushing times are insufficient, such as when you want to
make sure a database trigger gets run at a particular time.
"""
self._event.emit("flush")
# The _dirty list may change under us while we're running
# the flush hooks, so we cannot just simply loop over it
# once. To prevent infinite looping we keep track of which
# objects we've called the hook for using a `flushing` dict.
flushing = {}
while self._dirty:
(obj_info, obj) = self._dirty.popitem()
if obj_info not in flushing:
flushing[obj_info] = obj
self._run_hook(obj_info, "__storm_pre_flush__")
self._dirty = flushing
predecessors = {}
for (before_info, after_info), n in self._order.iteritems():
if n > 0:
before_set = predecessors.get(after_info)
if before_set is None:
predecessors[after_info] = set((before_info,))
else:
before_set.add(before_info)
key_func = itemgetter("sequence")
# The external loop is important because items can get into the dirty
# state while we're flushing objects, ...
while self._dirty:
# ... but we don't have to resort everytime an object is flushed,
# so we have an internal loop too. If no objects become dirty
# during flush, this will clean self._dirty and the external loop
# will exit too.
sorted_dirty = sorted(self._dirty, key=key_func)
while sorted_dirty:
for i, obj_info in enumerate(sorted_dirty):
for before_info in predecessors.get(obj_info, ()):
if before_info in self._dirty:
break # A predecessor is still dirty.
else:
break # Found an item without dirty predecessors.
else:
raise OrderLoopError("Can't flush due to ordering loop")
del sorted_dirty[i]
self._dirty.pop(obj_info, None)
self._flush_one(obj_info)
self._order.clear()
# That's not stricly necessary, but prevents getting into bigints.
self._sequence = 0
def _flush_one(self, obj_info):
cls_info = obj_info.cls_info
pending = obj_info.pop("pending", None)
if pending is PENDING_REMOVE:
expr = Delete(compare_columns(cls_info.primary_key,
obj_info["primary_vars"]),
cls_info.table)
self._connection.execute(expr, noresult=True)
# We're sure the cache is valid at this point.
obj_info.pop("invalidated", None)
self._disable_change_notification(obj_info)
self._remove_from_alive(obj_info)
del obj_info["store"]
elif pending is PENDING_ADD:
# Give a chance to the backend to process primary variables.
self._connection.preset_primary_key(cls_info.primary_key,
obj_info.primary_vars)
changes = self._get_changes_map(obj_info, True)
expr = Insert(changes, cls_info.table,
primary_columns=cls_info.primary_key,
primary_variables=obj_info.primary_vars)
result = self._connection.execute(expr)
# We're sure the cache is valid at this point. We just added
# the object.
obj_info.pop("invalidated", None)
self._fill_missing_values(obj_info, obj_info.primary_vars, result)
self._enable_change_notification(obj_info)
self._add_to_alive(obj_info)
else:
cached_primary_vars = obj_info["primary_vars"]
changes = self._get_changes_map(obj_info)
if changes:
expr = Update(changes,
compare_columns(cls_info.primary_key,
cached_primary_vars),
cls_info.table)
self._connection.execute(expr, noresult=True)
self._fill_missing_values(obj_info, obj_info.primary_vars)
self._add_to_alive(obj_info)
self._run_hook(obj_info, "__storm_flushed__")
obj_info.event.emit("flushed")
def block_implicit_flushes(self):
"""Block implicit flushes from operations like execute()."""
self._implicit_flush_block_count += 1
def unblock_implicit_flushes(self):
"""Unblock implicit flushes from operations like execute()."""
assert self._implicit_flush_block_count > 0
self._implicit_flush_block_count -= 1
def block_access(self):
"""Block access to the underlying database connection."""
self._connection.block_access()
def unblock_access(self):
"""Unblock access to the underlying database connection."""
self._connection.unblock_access()
def _get_changes_map(self, obj_info, adding=False):
"""Return a {column: variable} dictionary suitable for inserts/updates.
@param obj_info: ObjectInfo to inspect for changes.
@param adding: If true, any defined variables will be considered
a change and included in the returned map.
"""
cls_info = obj_info.cls_info
changes = {}
select_variables = []
for column in cls_info.columns:
variable = obj_info.variables[column]
if adding or variable.has_changed():
if variable.is_defined():
changes[column] = variable
else:
lazy_value = variable.get_lazy()
if isinstance(lazy_value, Expr):
if id(column) in cls_info.primary_key_idx:
select_variables.append(variable) # See below.
changes[column] = variable
else:
changes[column] = lazy_value
# If we have any expressions in the primary variables, we
# have to resolve them now so that we have the identity of
# the inserted object available later.
if select_variables:
resolve_expr = Select([variable.get_lazy()
for variable in select_variables])
result = self._connection.execute(resolve_expr)
for variable, value in zip(select_variables, result.get_one()):
result.set_variable(variable, value)
return changes
def _fill_missing_values(self, obj_info, primary_vars, result=None):
"""Fill missing values in variables of the given obj_info.
This method will verify which values are unset in obj_info,
and set them to AutoReload, or if it's part of the primary
key, query the database for the actual values.
@param obj_info: ObjectInfo to have its values filled.
@param primary_vars: Variables composing the primary key with
up-to-date values (cached variables may be out-of-date when
this method is called).
@param result: If some value in the set of primary variables
isn't defined, it must be retrieved from the database
using database-dependent logic, which is provided by the
backend in the result of the query which inserted the object.
"""
cls_info = obj_info.cls_info
cached_primary_vars = obj_info.get("primary_vars")
primary_key_idx = cls_info.primary_key_idx
missing_columns = []
for column in cls_info.columns:
variable = obj_info.variables[column]
if not variable.is_defined():
idx = primary_key_idx.get(id(column))
if idx is not None:
if (cached_primary_vars is not None
and variable.get_lazy() is AutoReload):
# For auto-reloading a primary key, just
# get the value out of the cache.
variable.set(cached_primary_vars[idx].get())
else:
missing_columns.append(column)
else:
# Any lazy values are overwritten here. This value
# must have just been sent to the database, so this
# was already set there.
variable.set(AutoReload)
else:
variable.checkpoint()
if missing_columns:
where = result.get_insert_identity(cls_info.primary_key,
primary_vars)
result = self._connection.execute(Select(missing_columns, where))
self._set_values(obj_info, missing_columns,
result, result.get_one())
def _validate_alive(self, obj_info):
"""Perform cache validation for the given obj_info."""
where = compare_columns(obj_info.cls_info.primary_key,
obj_info["primary_vars"])
result = self._connection.execute(Select(SQLRaw("1"), where))
if not result.get_one():
raise LostObjectError("Object is not in the database anymore")
obj_info.pop("invalidated", None)
def _load_object(self, cls_info, result, values):
# _set_values() need the cls_info columns for the class of the
# actual object, not from a possible wrapper (e.g. an alias).
cls = cls_info.cls
cls_info = get_cls_info(cls)
# Prepare cache key.
primary_vars = []
columns = cls_info.columns
for value in values:
if value is not None:
break
else:
# We've got a row full of NULLs, so consider that the object
# wasn't found. This is useful for joins, where non-existent
# rows are represented like that.
return None
for i in cls_info.primary_key_pos:
value = values[i]
variable = columns[i].variable_factory(value=value, from_db=True)
primary_vars.append(variable)
# Lookup cache.
primary_values = tuple(var.get(to_db=True) for var in primary_vars)
obj_info = self._alive.get((cls, primary_values))
if obj_info is not None:
# Found object in cache, and it must be valid since the
# primary key was extracted from result values.
obj_info.pop("invalidated", None)
# Take that chance and fill up any undefined variables
# with fresh data, since we got it anyway.
self._set_values(obj_info, cls_info.columns, result,
values, keep_defined=True)
# We're not sure if the obj is still in memory at this
# point. This will rebuild it if needed.
obj = self._get_object(obj_info)
else:
# Nothing found in the cache. Build everything from the ground.
obj = cls.__new__(cls)
obj_info = get_obj_info(obj)
obj_info["store"] = self
self._set_values(obj_info, cls_info.columns, result, values,
replace_unknown_lazy=True)
self._add_to_alive(obj_info)
self._enable_change_notification(obj_info)
self._enable_lazy_resolving(obj_info)
self._run_hook(obj_info, "__storm_loaded__")
return obj
def _get_object(self, obj_info):
"""Return object for obj_info, rebuilding it if it's dead."""
obj = obj_info.get_obj()
if obj is None:
cls = obj_info.cls_info.cls
obj = cls.__new__(cls)
obj_info.set_obj(obj)
set_obj_info(obj, obj_info)
# Re-enable change notification, as it may have been implicitely
# disabled when the previous object has been collected
self._enable_change_notification(obj_info)
self._run_hook(obj_info, "__storm_loaded__")
# Renew the cache.
self._cache.add(obj_info)
return obj
@staticmethod
def _run_hook(obj_info, hook_name):
func = getattr(obj_info.get_obj(), hook_name, None)
if func is not None:
func()
def _set_values(self, obj_info, columns, result, values,
keep_defined=False, replace_unknown_lazy=False):
if values is None:
raise LostObjectError("Can't obtain values from the database "
"(object got removed?)")
obj_info.pop("invalidated", None)
for column, value in zip(columns, values):
variable = obj_info.variables[column]
lazy_value = variable.get_lazy()
is_unknown_lazy = not (lazy_value is None or
lazy_value is AutoReload)
if keep_defined:
if variable.is_defined() or is_unknown_lazy:
continue
elif is_unknown_lazy and not replace_unknown_lazy:
# This should *never* happen, because whenever we get
# to this point it should be after a flush() which
# updated the database with lazy values and then replaced
# them by AutoReload. Letting this go through means
# we're blindly discarding an unknown lazy value and
# replacing it by the value from the database.
raise RuntimeError("Unexpected situation. "
"Please contact the developers.")
if value is None:
variable.set(value, from_db=True)
else:
result.set_variable(variable, value)
variable.checkpoint()
def _is_dirty(self, obj_info):
return obj_info in self._dirty
def _set_dirty(self, obj_info):
if obj_info not in self._dirty:
self._dirty[obj_info] = obj_info.get_obj()
obj_info["sequence"] = self._sequence = self._sequence + 1
def _set_clean(self, obj_info):
self._dirty.pop(obj_info, None)
def _iter_dirty(self):
return self._dirty
def _add_to_alive(self, obj_info):
"""Add an object to the set of known in-memory objects.
When an object is added to the set of known in-memory objects,
the key is built from a copy of the current variables that are
part of the primary key. This means that, when an object is
retrieved from the database, these values may be used to get
the cached object which is already in memory, even if it
requested the primary key value to be changed. For that reason,
when changes to the primary key are flushed, the alive object
key should also be updated to reflect these changes.
In addition to tracking objects alive in memory, we have a strong
reference cache which keeps a fixed number of last-used objects
in-memory, to prevent further database access for recently fetched
objects.
"""
cls_info = obj_info.cls_info
old_primary_vars = obj_info.get("primary_vars")
if old_primary_vars is not None:
old_primary_values = tuple(
var.get(to_db=True) for var in old_primary_vars)
self._alive.pop((cls_info.cls, old_primary_values), None)
new_primary_vars = tuple(variable.copy()
for variable in obj_info.primary_vars)
new_primary_values = tuple(
var.get(to_db=True) for var in new_primary_vars)
self._alive[cls_info.cls, new_primary_values] = obj_info
obj_info["primary_vars"] = new_primary_vars
self._cache.add(obj_info)
def _remove_from_alive(self, obj_info):
"""Remove an object from the cache.
This method is only called for objects that were explicitly
deleted and flushed. Objects that are unused will get removed
from the cache dictionary automatically by their weakref callbacks.
"""
primary_vars = obj_info.get("primary_vars")
if primary_vars is not None:
self._cache.remove(obj_info)
primary_values = tuple(var.get(to_db=True) for var in primary_vars)
del self._alive[obj_info.cls_info.cls, primary_values]
del obj_info["primary_vars"]
def _iter_alive(self):
return self._alive.values()
def _enable_change_notification(self, obj_info):
obj_info.event.emit("start-tracking-changes", self._event)
obj_info.event.hook("changed", self._variable_changed)
def _disable_change_notification(self, obj_info):
obj_info.event.unhook("changed", self._variable_changed)
obj_info.event.emit("stop-tracking-changes", self._event)
def _variable_changed(self, obj_info, variable,
old_value, new_value, fromdb):
# The fromdb check makes sure that values coming from the
# database don't mark the object as dirty again.
# XXX The fromdb check is untested. How to test it?
if not fromdb:
if new_value is not Undef and new_value is not AutoReload:
if obj_info.get("invalidated"):
# This might be a previously alive object being
# updated. Let's validate it now to improve debugging.
# This will raise LostObjectError if the object is gone.
self._validate_alive(obj_info)
self._set_dirty(obj_info)
def _enable_lazy_resolving(self, obj_info):
obj_info.event.hook("resolve-lazy-value", self._resolve_lazy_value)
def _disable_lazy_resolving(self, obj_info):
obj_info.event.unhook("resolve-lazy-value", self._resolve_lazy_value)
def _resolve_lazy_value(self, obj_info, variable, lazy_value):
"""Resolve a variable set to a lazy value when it's touched.
This method is hooked into the obj_info to resolve variables
set to lazy values when they're accessed. It will first flush
the store, and then set all variables set to AutoReload to
their database values.
"""
if lazy_value is not AutoReload and not isinstance(lazy_value, Expr):
# It's not something we handle.
return
# XXX This will do it for now, but it should really flush
# just this single object and ones that it depends on.
# _flush_one() doesn't consider dependencies, so it may
# not be used directly. Maybe allow flush(obj)?
if self._implicit_flush_block_count == 0:
self.flush()
autoreload_columns = []
for column in obj_info.cls_info.columns:
if obj_info.variables[column].get_lazy() is AutoReload:
autoreload_columns.append(column)
if autoreload_columns:
where = compare_columns(obj_info.cls_info.primary_key,
obj_info["primary_vars"])
result = self._connection.execute(
Select(autoreload_columns, where))
self._set_values(obj_info, autoreload_columns,
result, result.get_one())
class ResultSet(object):
"""The representation of the results of a query.
Note that having an instance of this class does not indicate that
a database query has necessarily been made. Database queries are
put off until absolutely necessary.
Generally these should not be constructed directly, but instead
retrieved from calls to L{Store.find}.
"""
def __init__(self, store, find_spec,
where=Undef, tables=Undef, select=Undef):
self._store = store
self._find_spec = find_spec
self._where = where
self._tables = tables
self._select = select
self._order_by = find_spec.default_order
self._offset = Undef
self._limit = Undef
self._distinct = False
self._group_by = Undef
self._having = Undef
def copy(self):
"""Return a copy of this ResultSet object, with the same configuration.
"""
result_set = object.__new__(self.__class__)
result_set.__dict__.update(self.__dict__)
if self._select is not Undef:
# This expression must be copied because we may have to change it
# in-place inside _get_select().
result_set._select = copy(self._select)
return result_set
def config(self, distinct=None, offset=None, limit=None):
"""Configure this result object in-place. All parameters are optional.
@param distinct: If True, enables usage of the DISTINCT keyword in
the query. If a tuple or list of columns, inserts a DISTINCT ON
(only supported by PostgreSQL).
@param offset: Offset where results will start to be retrieved
from the result set.
@param limit: Limit the number of objects retrieved from the
result set.
@return: self (not a copy).
"""
if distinct is not None:
self._distinct = distinct
if offset is not None:
self._offset = offset
if limit is not None:
self._limit = limit
return self
def _get_select(self):
if self._select is not Undef:
if self._order_by is not Undef:
self._select.order_by = self._order_by
if self._limit is not Undef: # XXX UNTESTED!
self._select.limit = self._limit
if self._offset is not Undef: # XXX UNTESTED!
self._select.offset = self._offset
return self._select
columns, default_tables = self._find_spec.get_columns_and_tables()
return Select(columns, self._where, self._tables, default_tables,
self._order_by, offset=self._offset, limit=self._limit,
distinct=self._distinct, group_by=self._group_by,
having=self._having)
def _load_objects(self, result, values):
return self._find_spec.load_objects(self._store, result, values)
def __iter__(self):
"""Iterate the results of the query.
"""
result = self._store._connection.execute(self._get_select())
for values in result:
yield self._load_objects(result, values)
def __getitem__(self, index):
"""Get an individual item by offset, or a range of items by slice.
@return: The matching object or, if a slice is used, a new
L{ResultSet} will be returned appropriately modified with
C{OFFSET} and C{LIMIT} clauses.
"""
if isinstance(index, (int, long)):
if index == 0:
result_set = self
else:
if self._offset is not Undef:
index += self._offset
result_set = self.copy()
result_set.config(offset=index, limit=1)
obj = result_set._any()
if obj is None:
raise IndexError("Index out of range")
return obj
if not isinstance(index, slice):
raise IndexError("Can't index ResultSets with %r" % (index,))
if index.step is not None:
raise IndexError("Stepped slices not yet supported: %r"
% (index.step,))
offset = self._offset
limit = self._limit
if index.start is not None:
if offset is Undef:
offset = index.start
else:
offset += index.start
if limit is not Undef:
limit = max(0, limit - index.start)
if index.stop is not None:
if index.start is None:
new_limit = index.stop
else:
new_limit = index.stop - index.start
if limit is Undef or limit > new_limit:
limit = new_limit
return self.copy().config(offset=offset, limit=limit)
def __contains__(self, item):
"""Check if an item is contained within the result set."""
columns, values = self._find_spec.get_columns_and_values_for_item(item)
if self._select is Undef and self._group_by is Undef:
# No predefined select: adjust the where clause.
dummy, default_tables = self._find_spec.get_columns_and_tables()
where = [Eq(*pair) for pair in zip(columns, values)]
if self._where is not Undef:
where.append(self._where)
select = Select(1, And(*where), self._tables,
default_tables)
else:
# Rewrite the predefined query and use it as a subquery.
aliased_columns = [Alias(column, "_key%d" % index)
for (index, column) in enumerate(columns)]
subquery = replace_columns(self._get_select(), aliased_columns)
where = [Eq(*pair) for pair in zip(aliased_columns, values)]
select = Select(1, And(*where), Alias(subquery, "_tmp"))
result = self._store._connection.execute(select)
return result.get_one() is not None
def is_empty(self):
"""Return C{True} if this result set doesn't contain any results."""
subselect = self._get_select()
subselect.limit = 1
subselect.order_by = Undef
select = Select(1, tables=Alias(subselect, "_tmp"), limit=1)
result = self._store._connection.execute(select)
return (not result.get_one())
def any(self):
"""Return a single item from the result set.
@return: An arbitrary object or C{None} if one isn't available.
@seealso: one(), first(), and last().
"""
select = self._get_select()
select.limit = 1
select.order_by = Undef
result = self._store._connection.execute(select)
values = result.get_one()
if values:
return self._load_objects(result, values)
return None
def _any(self):
"""Return a single item from the result without changing sort order.
@return: An arbitrary object or C{None} if one isn't available.
"""
select = self._get_select()
select.limit = 1
result = self._store._connection.execute(select)
values = result.get_one()
if values:
return self._load_objects(result, values)
return None
def first(self):
"""Return the first item from an ordered result set.
@raises UnorderedError: Raised if the result set isn't ordered.
@return: The first object or C{None} if one isn't available.
@seealso: last(), one(), and any().
"""
if self._order_by is Undef:
raise UnorderedError("Can't use first() on unordered result set")
return self._any()
def last(self):
"""Return the last item from an ordered result set.
@raises FeatureError: Raised if the result set has a C{LIMIT} set.
@raises UnorderedError: Raised if the result set isn't ordered.
@return: The last object or C{None} if one isn't available.
@seealso: first(), one(), and any().
"""
if self._order_by is Undef:
raise UnorderedError("Can't use last() on unordered result set")
if self._limit is not Undef:
raise FeatureError("Can't use last() with a slice "
"of defined stop index")
select = self._get_select()
select.offset = Undef
select.limit = 1
select.order_by = []
for expr in self._order_by:
if isinstance(expr, Desc):
select.order_by.append(expr.expr)
elif isinstance(expr, Asc):
select.order_by.append(Desc(expr.expr))
else:
select.order_by.append(Desc(expr))
result = self._store._connection.execute(select)
values = result.get_one()
if values:
return self._load_objects(result, values)
return None
def one(self):
"""Return one item from a result set containing at most one item.
@raises NotOneError: Raised if the result set contains more than one
item.
@return: The object or C{None} if one isn't available.
@seealso: first(), one(), and any().
"""
select = self._get_select()
# limit could be 1 due to slicing, for instance.
if select.limit is not Undef and select.limit > 2:
select.limit = 2
result = self._store._connection.execute(select)
values = result.get_one()
if result.get_one():
raise NotOneError("one() used with more than one result available")
if values:
return self._load_objects(result, values)
return None
def order_by(self, *args):
"""Specify the ordering of the results.
The query will be modified appropriately with an ORDER BY clause.
Ascending and descending order can be specified by wrapping
the columns in L{Asc} and L{Desc}.
@param args: One or more L{storm.expr.Column} objects.
"""
if self._offset is not Undef or self._limit is not Undef:
raise FeatureError("Can't reorder a sliced result set")
self._order_by = args or Undef
return self
def remove(self):
"""Remove all rows represented by this ResultSet from the database.
This is done efficiently with a DELETE statement, so objects
are not actually loaded into Python.
"""
if self._group_by is not Undef:
raise FeatureError("Removing isn't supported after a "
" GROUP BY clause ")
if self._offset is not Undef or self._limit is not Undef:
raise FeatureError("Can't remove a sliced result set")
if self._find_spec.default_cls_info is None:
raise FeatureError("Removing not yet supported for tuple or "
"expression finds")
if self._select is not Undef:
raise FeatureError("Removing isn't supported with "
"set expressions (unions, etc)")
result = self._store._connection.execute(
Delete(self._where, self._find_spec.default_cls_info.table))
return result.rowcount
def group_by(self, *expr):
"""Group this ResultSet by the given expressions.
@param expr: The expressions used in the GROUP BY statement.
@return: self (not a copy).
"""
if self._select is not Undef:
raise FeatureError("Grouping isn't supported with "
"set expressions (unions, etc)")
find_spec = FindSpec(expr)
columns, dummy = find_spec.get_columns_and_tables()
self._group_by = columns
return self
def having(self, *expr):
"""Filter result previously grouped by.
@param expr: Instances of L{Expr}.
@return: self (not a copy).
"""
if self._group_by is Undef:
raise FeatureError("having can only be called after group_by.")
self._having = And(*expr)
return self
def _aggregate(self, aggregate_func, expr, column=None):
if self._group_by is not Undef:
raise FeatureError("Single aggregates aren't supported after a "
" GROUP BY clause ")
columns, default_tables = self._find_spec.get_columns_and_tables()
if (self._select is Undef and not self._distinct and
self._offset is Undef and self._limit is Undef):
select = Select(aggregate_func(expr), self._where,
self._tables, default_tables)
else:
if expr is Undef:
aggregate = aggregate_func(expr)
else:
alias = Alias(expr, "_expr")
columns.append(alias)
aggregate = aggregate_func(alias)
subquery = replace_columns(self._get_select(), columns)
select = Select(aggregate, tables=Alias(subquery, "_tmp"))
result = self._store._connection.execute(select)
value = result.get_one()[0]
variable_factory = getattr(column, "variable_factory", None)
if variable_factory:
variable = variable_factory(allow_none=True)
result.set_variable(variable, value)
return variable.get()
return value
def count(self, expr=Undef, distinct=False):
"""Get the number of objects represented by this ResultSet."""
return int(self._aggregate(lambda expr: Count(expr, distinct), expr))
def max(self, expr):
"""Get the highest value from an expression."""
return self._aggregate(Max, expr, expr)
def min(self, expr):
"""Get the lowest value from an expression."""
return self._aggregate(Min, expr, expr)
def avg(self, expr):
"""Get the average value from an expression."""
value = self._aggregate(Avg, expr)
if value is None:
return value
return float(value)
def sum(self, expr):
"""Get the sum of all values in an expression."""
return self._aggregate(Sum, expr, expr)
def get_select_expr(self, *columns):
"""Get a L{Select} expression to retrieve only the specified columns.
@param columns: One or more L{storm.expr.Column} objects whose values
will be fetched.
@raises FeatureError: Raised if no columns are specified or if this
result is a set expression such as a union.
@return: A L{Select} expression configured to use the query parameters
specified for this result set, and also limited to only retrieving
data for the specified columns.
"""
if not columns:
raise FeatureError("select() takes at least one column "
"as argument")
if self._select is not Undef:
raise FeatureError(
"Can't generate subselect expression for set expressions")
select = self._get_select()
select.columns = columns
return select
def values(self, *columns):
"""Retrieve only the specified columns.
This does not load full objects from the database into Python.
@param columns: One or more L{storm.expr.Column} objects whose
values will be fetched.
@raises FeatureError: Raised if no columns are specified or if this
result is a set expression such as a union.
@return: An iterator of tuples of the values for each column
from each matching row in the database.
"""
if not columns:
raise FeatureError("values() takes at least one column "
"as argument")
if self._select is not Undef:
raise FeatureError("values() can't be used with set expressions")
select = self._get_select()
select.columns = columns
result = self._store._connection.execute(select)
if len(columns) == 1:
variable = columns[0].variable_factory()
for values in result:
result.set_variable(variable, values[0])
yield variable.get()
else:
variables = [column.variable_factory() for column in columns]
for values in result:
for variable, value in zip(variables, values):
result.set_variable(variable, value)
yield tuple(variable.get() for variable in variables)
def set(self, *args, **kwargs):
"""Update objects in the result set with the given arguments.
This method will update all objects in the current result set
to match expressions given as equalities or keyword arguments.
These objects may still be in the database (an UPDATE is issued)
or may be cached.
For instance, C{result.set(Class.attr1 == 1, attr2=2)} will set
C{attr1} to 1 and C{attr2} to 2, on all matching objects.
"""
if self._group_by is not Undef:
raise FeatureError("Setting isn't supported after a "
" GROUP BY clause ")
if self._find_spec.default_cls_info is None:
raise FeatureError("Setting isn't supported with tuple or "
"expression finds")
if self._select is not Undef:
raise FeatureError("Setting isn't supported with "
"set expressions (unions, etc)")
if not (args or kwargs):
return
changes = {}
cls = self._find_spec.default_cls_info.cls
# For now only "Class.attr == var" is supported in args.
for expr in args:
if not isinstance(expr, Eq):
raise FeatureError("Unsupported set expression: %r" %
repr(expr))
elif not isinstance(expr.expr1, Column):
raise FeatureError("Unsupported left operand in set "
"expression: %r" % repr(expr.expr1))
elif not isinstance(expr.expr2, (Expr, Variable)):
raise FeatureError("Unsupported right operand in set "
"expression: %r" % repr(expr.expr2))
changes[expr.expr1] = expr.expr2
for key, value in kwargs.items():
column = getattr(cls, key)
if value is None:
changes[column] = None
elif isinstance(value, Expr):
changes[column] = value
else:
changes[column] = column.variable_factory(value=value)
expr = Update(changes, self._where,
self._find_spec.default_cls_info.table)
self._store.execute(expr, noresult=True)
try:
cached = self.cached()
except CompileError:
# We are iterating through all objects in memory here, so
# check if the object type matches to avoid trying to
# invalidate a column that does not exist, on an unrelated
# object.
for obj_info in self._store._iter_alive():
if obj_info.cls_info is self._find_spec.default_cls_info:
for column in changes:
obj_info.variables[column].set(AutoReload)
else:
changes = changes.items()
for obj in cached:
for column, value in changes:
variables = get_obj_info(obj).variables
if value is None:
pass
elif isinstance(value, Variable):
value = value.get()
elif isinstance(value, Expr):
# If the value is an Expression that means we
# can't compute it by ourselves: we rely on
# the database to compute it, so just set the
# value to AutoReload.
value = AutoReload
else:
value = variables[value].get()
variables[column].set(value)
variables[column].checkpoint()
def cached(self):
"""Return matching objects from the cache for the current query."""
if self._find_spec.default_cls_info is None:
raise FeatureError("Cache finds not supported with tuples "
"or expressions")
if self._tables is not Undef:
raise FeatureError("Cache finds not supported with custom tables")
if self._where is Undef:
match = None
else:
match = compile_python.get_matcher(self._where)
def get_column(column):
return obj_info.variables[column].get()
objects = []
for obj_info in self._store._iter_alive():
try:
if (obj_info.cls_info is self._find_spec.default_cls_info and
(match is None or match(get_column))):
objects.append(self._store._get_object(obj_info))
except LostObjectError:
pass # This may happen when resolving lazy values
# in get_column().
return objects
def find(self, *args, **kwargs):
"""Perform a query on objects within this result set.
This is analogous to L{Store.find}, although it doesn't take a
C{cls_spec} argument, instead using the same tables as the
existing result set, and restricts the results to those in
this set.
@param args: Instances of L{Expr}.
@param kwargs: Mapping of simple column names to values or
expressions to query for.
@return: A L{ResultSet} of matching instances.
"""
if self._select is not Undef:
raise FeatureError("Can't query set expressions")
if self._offset is not Undef or self._limit is not Undef:
raise FeatureError("Can't query a sliced result set")
if self._group_by is not Undef:
raise FeatureError("Can't query grouped result sets")
result_set = self.copy()
extra_where = get_where_for_args(
args, kwargs, self._find_spec.default_cls)
if extra_where is not Undef:
if result_set._where is Undef:
result_set._where = extra_where
else:
result_set._where = And(result_set._where, extra_where)
return result_set
def _set_expr(self, expr_cls, other, all=False):
if not self._find_spec.is_compatible(other._find_spec):
raise FeatureError("Incompatible results for set operation")
expr = expr_cls(self._get_select(), other._get_select(), all=all)
return ResultSet(self._store, self._find_spec, select=expr)
def union(self, other, all=False):
"""Get the L{Union} of this result set and another.
@param all: If True, include duplicates.
"""
if isinstance(other, EmptyResultSet):
return self
return self._set_expr(Union, other, all)
def difference(self, other, all=False):
"""Get the difference, using L{Except}, of this result set and another.
@param all: If True, include duplicates.
"""
if isinstance(other, EmptyResultSet):
return self
return self._set_expr(Except, other, all)
def intersection(self, other, all=False):
"""Get the L{Intersection} of this result set and another.
@param all: If True, include duplicates.
"""
if isinstance(other, EmptyResultSet):
return other
return self._set_expr(Intersect, other, all)
class EmptyResultSet(object):
"""An object that looks like a L{ResultSet} but represents no rows.
This is convenient for application developers who want to provide
a method which is guaranteed to return a L{ResultSet}-like object
but which, in certain cases, knows there is no point in querying
the database. For example::
def get_people(self, ids):
if not ids:
return EmptyResultSet()
return store.find(People, People.id.is_in(ids))
The methods on EmptyResultSet (L{one}, L{config}, L{union}, etc)
are meant to emulate a L{ResultSet} which has matched no rows.
"""
def __init__(self, ordered=False):
self._order_by = ordered
def copy(self):
result = EmptyResultSet(self._order_by)
return result
def config(self, distinct=None, offset=None, limit=None):
pass
def __iter__(self):
return
yield None
def __getitem__(self, index):
return self.copy()
def __contains__(self, item):
return False
def is_empty(self):
return True
def any(self):
return None
def first(self):
if self._order_by:
return None
raise UnorderedError("Can't use first() on unordered result set")
def last(self):
if self._order_by:
return None
raise UnorderedError("Can't use last() on unordered result set")
def one(self):
return None
def order_by(self, *args):
self._order_by = True
return self
def remove(self):
return 0
def count(self, expr=Undef, distinct=False):
return 0
def max(self, column):
return None
def min(self, column):
return None
def avg(self, column):
return None
def sum(self, column):
return None
def get_select_expr(self, *columns):
"""Get a L{Select} expression to retrieve only the specified columns.
@param columns: One or more L{storm.expr.Column} objects whose values
will be fetched.
@raises FeatureError: Raised if no columns are specified.
@return: A L{Select} expression configured to use the query parameters
specified for this result set. The result of the select will
always be an empty set of rows.
"""
if not columns:
raise FeatureError("select() takes at least one column "
"as argument")
return Select(columns, False)
def values(self, *columns):
if not columns:
raise FeatureError("values() takes at least one column "
"as argument")
return
yield None
def set(self, *args, **kwargs):
pass
def cached(self):
return []
def find(self, *args, **kwargs):
return self
def union(self, other):
if isinstance(other, EmptyResultSet):
return self
return other.union(self)
def difference(self, other):
return self
def intersection(self, other):
return self
class TableSet(object):
"""The representation of a set of tables which can be queried at once.
This will typically be constructed by a call to L{Store.using}.
"""
def __init__(self, store, tables):
self._store = store
self._tables = tables
def find(self, cls_spec, *args, **kwargs):
"""Perform a query on the previously specified tables.
This is identical to L{Store.find} except that the tables are
explicitly specified instead of relying on inference.
@return: A L{ResultSet}.
"""
if self._store._implicit_flush_block_count == 0:
self._store.flush()
find_spec = FindSpec(cls_spec)
where = get_where_for_args(args, kwargs, find_spec.default_cls)
return self._store._result_set_factory(self._store, find_spec,
where, self._tables)
Store._result_set_factory = ResultSet
Store._table_set = TableSet
class FindSpec(object):
"""The set of tables or expressions in the result of L{Store.find}."""
def __init__(self, cls_spec):
self.is_tuple = type(cls_spec) == tuple
if not self.is_tuple:
cls_spec = (cls_spec,)
info = []
for item in cls_spec:
if isinstance(item, Expr):
info.append((True, item))
else:
info.append((False, get_cls_info(item)))
self._cls_spec_info = tuple(info)
# Do we have a single non-expression item here?
if not self.is_tuple and not info[0][0]:
self.default_cls = cls_spec[0]
self.default_cls_info = info[0][1]
self.default_order = self.default_cls_info.default_order
else:
self.default_cls = None
self.default_cls_info = None
self.default_order = Undef
def get_columns_and_tables(self):
columns = []
default_tables = []
for is_expr, info in self._cls_spec_info:
if is_expr:
columns.append(info)
if isinstance(info, Column):
default_tables.append(info.table)
else:
columns.extend(info.columns)
default_tables.append(info.table)
return columns, default_tables
def is_compatible(self, find_spec):
"""Return True if this FindSpec is compatible with a second one."""
if self.is_tuple != find_spec.is_tuple:
return False
if len(self._cls_spec_info) != len(find_spec._cls_spec_info):
return False
for (is_expr1, info1), (is_expr2, info2) in zip(
self._cls_spec_info, find_spec._cls_spec_info):
if is_expr1 != is_expr2:
return False
if info1 is not info2:
return False
return True
def load_objects(self, store, result, values):
objects = []
values_start = values_end = 0
for is_expr, info in self._cls_spec_info:
if is_expr:
values_end += 1
variable = getattr(info, "variable_factory", Variable)(
value=values[values_start], from_db=True)
objects.append(variable.get())
else:
values_end += len(info.columns)
obj = store._load_object(info, result,
values[values_start:values_end])
objects.append(obj)
values_start = values_end
if self.is_tuple:
return tuple(objects)
else:
return objects[0]
def get_columns_and_values_for_item(self, item):
"""Generate a comparison expression with the given item."""
if isinstance(item, tuple):
if not self.is_tuple:
raise TypeError("Find spec does not expect tuples.")
else:
if self.is_tuple:
raise TypeError("Find spec expects tuples.")
item = (item,)
columns = []
values = []
for (is_expr, info), value in zip(self._cls_spec_info, item):
if is_expr:
if not isinstance(value, (Expr, Variable)) and (
value is not None):
value = getattr(info, "variable_factory", Variable)(
value=value)
columns.append(info)
values.append(value)
else:
obj_info = get_obj_info(value)
if obj_info.cls_info != info:
raise TypeError("%r does not match %r" % (value, info))
columns.extend(info.primary_key)
values.extend(obj_info.primary_vars)
return columns, values
def get_where_for_args(args, kwargs, cls=None):
equals = list(args)
if kwargs:
if cls is None:
raise FeatureError("Can't determine class that keyword "
"arguments are associated with")
for key, value in kwargs.items():
equals.append(getattr(cls, key) == value)
if equals:
return And(*equals)
return Undef
def replace_columns(expr, columns):
if isinstance(expr, Select):
select = copy(expr)
select.columns = columns
# Remove the ordering if it won't affect the result of the query.
if select.limit is Undef and select.offset is Undef:
select.order_by = Undef
return select
elif isinstance(expr, SetExpr):
# The ORDER BY clause might refer to columns we have replaced.
# Luckily we can ignore it if there is no limit/offset.
if expr.order_by is not Undef and (
expr.limit is not Undef or expr.offset is not Undef):
raise FeatureError(
"__contains__() does not yet support set "
"expressions that combine ORDER BY with "
"LIMIT/OFFSET")
subexprs = [replace_columns(subexpr, columns)
for subexpr in expr.exprs]
return expr.__class__(
all=expr.all, limit=expr.limit, offset=expr.offset,
*subexprs)
else:
raise FeatureError(
"__contains__() does not yet support %r expressions"
% (expr.__class__,))
class AutoReload(LazyValue):
"""A marker for reloading a single value.
Often this will be used to specify that a specific attribute
should be loaded from the database on the next access, like so::
storm_object.property = AutoReload
On the next access to C{storm_object.property}, the value will be
loaded from the database.
It is also often used as a default value for a property::
class Person(object):
__storm_table__ = "person"
id = Int(allow_none=False, default=AutoReload)
person = store.add(Person)
person.id # gets the attribute from the database.
"""
pass
AutoReload = AutoReload()
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