/usr/share/GNUstep/Documentation/GDL2Intro/EOAttribute-class.html

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<a name="EOAttribute-class"></a>
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Next: <a href="EORelationship-class.html#EORelationship-class" accesskey="n" rel="next">EORelationship class</a>, Previous: <a href="EOEntity-class.html#EOEntity-class" accesskey="p" rel="prev">EOEntity class</a>, Up: <a href="Classes.html#Classes" accesskey="u" rel="up">Classes</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="EOAttribute-class-1"></a>
<h2 class="chapter">15 EOAttribute class</h2>
<a name="index-class_002c-EOAttribute"></a>

<a name="overview-2"></a>
<h3 class="section">15.1 overview</h3>
<p>An attribute typically maps a table column to an instance variable, 
in which case the attribute is a class property.  Some attributes 
represent foreign keys which are used to create realationships yet do 
not correspond to a property in the enterprise object.  Other attributes 
may represent primary keys which needn&rsquo;t be class property either.  In fact 
some parts of framework work more smoothly if primary key attributes and 
foreign key attributes are not class properties.
</p>
<p>Attributes typically contain:
</p><ol>
<li> A name
</li><li> A column name
</li><li> An adaptor value type
</li><li> An external type
</li><li> A value type
</li><li> A value class name
</li><li> A value factory method name
</li><li> a factory method argument type
</li></ol>

<p>Some additional properties an attribute may have:
</p><ol>
<li> Read only
</li><li> Allows null
</li><li> Width
</li><li> Precision
</li><li> Scale
</li></ol>

<a name="Name"></a>
<h3 class="section">15.2 Name</h3>
<p>The attributes name when the attribute is a class property is used
as the key when doing key value coding on an enterprise object.
</p>
<p>It also uniquely identifies the attribute in its entity
there many not be an attribute with the same name as another attribute or 
relationship in an entity.
</p>
<a name="Column-name"></a>
<h3 class="section">15.3 Column name</h3>
<p>The adaptor uses the column name in generating SQL.
</p>
<a name="Adaptor-value-type"></a>
<h3 class="section">15.4 Adaptor value type</h3>
<p>Indicates the type of the attribute as contained in the database
</p>
<p>Valid adaptor value types are:
</p><ol>
<li> EOAdaptorNumberType
</li><li> EOAdaptorCharactersType
</li><li> EOAdaptorBytesType
</li><li> EOAdaptorDateType
</li></ol>

<p>Corresponding to numerical, string, raw data, and date value types.
</p>
<a name="External-type"></a>
<h3 class="section">15.5 External type</h3>
<p>An external type is a string representing an adaptor specific database type
different adaptors may use different names
where the PostgreSQL adaptor might use &rsquo;char&rsquo;.
The SQLite3 Adaptor might use &rsquo;TEXT&rsquo;
</p>
<p>it gives you full control on how the data is stored in the specific adaptor
where the adaptor value type allows you to specify a few generic values. 
</p>
<a name="Value-type"></a>
<h3 class="section">15.6 Value type</h3>
<p>Value types are a string with a single character such as &rsquo;f&rsquo; for floats
&rsquo;c&rsquo; for chars a full list of the standard types is available in the 
GDL2 API reference for EOAttributes -valueType method.
</p>
<p>The value type allows you to further refine the adaptor value type
where EOAdaptorNumberType might represent a integer, float, or double type.
</p>
<a name="Value-class-name"></a>
<h3 class="section">15.7 Value class name</h3>
<p>The value class name specifies the class which will be present in an
Enterprise Object containing the attribute.
</p>
<p>A property of this class will be instantiated when a field is retrieved from
the database, similarly a instance of this will be converted into the
external type when being sent to the datbase server.
</p>
<a name="Value-factory-method-name"></a>
<h3 class="section">15.8 Value factory method name</h3>
<p>When the Value Class Name is a custom object for instance NSImage
created from a blob of data. The value factory method name denotes
the initializer for the class, used to create a new instance of the custom
class.
</p>
<p>The value class name is an NSString representing a selector accepting a single
argument suitable for passing to the NSSelectorFromString function.
</p>
<a name="Value-factory-argument-type"></a>
<h3 class="section">15.9 Value factory argument type</h3>
<p>This is the type of the argument sent to the value factory method name.
</p>
<p>Valid types are
</p><ol>
<li> EOFactoryMethodArgumentIsNSData
</li><li> EOFactoryMethodArgumentIsNSString
</li><li> EOFactoryMethodArgumentIsBytes
</li></ol>



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