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  <div class="section" id="aggregate-fields">
<h1>Aggregate Fields<a class="headerlink" href="#aggregate-fields" title="Permalink to this headline"></a></h1>
<p><em>Section author: Benjamin Eberlei &lt;<a class="reference external" href="mailto:kontakt&#37;&#52;&#48;beberlei&#46;de">kontakt<span>&#64;</span>beberlei<span>&#46;</span>de</a>&gt;</em></p>
<p>You will often come across the requirement to display aggregate
values of data that can be computed by using the MIN, MAX, COUNT or
SUM SQL functions. For any ORM this is a tricky issue
traditionally. Doctrine 2 offers several ways to get access to
these values and this article will describe all of them from
different perspectives.</p>
<p>You will see that aggregate fields can become very explicit
features in your domain model and how this potentially complex
business rules can be easily tested.</p>
<div class="section" id="an-example-model">
<h2>An example model<a class="headerlink" href="#an-example-model" title="Permalink to this headline"></a></h2>
<p>Say you want to model a bank account and all their entries. Entries
into the account can either be of positive or negative money
values. Each account has a credit limit and the account is never
allowed to have a balance below that value.</p>
<p>For simplicity we live in a world were money is composed of
integers only. Also we omit the receiver/sender name, stated reason
for transfer and the execution date. These all would have to be
added on the <tt class="docutils literal"><span class="pre">Entry</span></tt> object.</p>
<p>Our entities look like:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">namespace</span> <span class="nx">Bank\Entities</span><span class="p">;</span>

<span class="sd">/**</span>
<span class="sd"> * @Entity</span>
<span class="sd"> */</span>
<span class="k">class</span> <span class="nc">Account</span>
<span class="p">{</span>
    <span class="sd">/** @Id @GeneratedValue @Column(type=&quot;integer&quot;) */</span>
    <span class="k">private</span> <span class="nv">$id</span><span class="p">;</span>

    <span class="sd">/** @Column(type=&quot;string&quot;, unique=true) */</span>
    <span class="k">private</span> <span class="nv">$no</span><span class="p">;</span>

    <span class="sd">/**</span>
<span class="sd">     * @OneToMany(targetEntity=&quot;Entry&quot;, mappedBy=&quot;account&quot;, cascade={&quot;persist&quot;})</span>
<span class="sd">     */</span>
    <span class="k">private</span> <span class="nv">$entries</span><span class="p">;</span>

    <span class="sd">/**</span>
<span class="sd">     * @Column(type=&quot;integer&quot;)</span>
<span class="sd">     */</span>
    <span class="k">private</span> <span class="nv">$maxCredit</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">__construct</span><span class="p">(</span><span class="nv">$no</span><span class="p">,</span> <span class="nv">$maxCredit</span> <span class="o">=</span> <span class="mi">0</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">no</span> <span class="o">=</span> <span class="nv">$no</span><span class="p">;</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">maxCredit</span> <span class="o">=</span> <span class="nv">$maxCredit</span><span class="p">;</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">entries</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">\Doctrine\Common\Collections\ArrayCollection</span><span class="p">();</span>
    <span class="p">}</span>
<span class="p">}</span>

<span class="sd">/**</span>
<span class="sd"> * @Entity</span>
<span class="sd"> */</span>
<span class="k">class</span> <span class="nc">Entry</span>
<span class="p">{</span>
    <span class="sd">/** @Id @GeneratedValue @Column(type=&quot;integer&quot;) */</span>
    <span class="k">private</span> <span class="nv">$id</span><span class="p">;</span>

    <span class="sd">/**</span>
<span class="sd">     * @ManyToOne(targetEntity=&quot;Account&quot;, inversedBy=&quot;entries&quot;)</span>
<span class="sd">     */</span>
    <span class="k">private</span> <span class="nv">$account</span><span class="p">;</span>

    <span class="sd">/**</span>
<span class="sd">     * @Column(type=&quot;integer&quot;)</span>
<span class="sd">     */</span>
    <span class="k">private</span> <span class="nv">$amount</span><span class="p">;</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">__construct</span><span class="p">(</span><span class="nv">$account</span><span class="p">,</span> <span class="nv">$amount</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">account</span> <span class="o">=</span> <span class="nv">$account</span><span class="p">;</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">amount</span> <span class="o">=</span> <span class="nv">$amount</span><span class="p">;</span>
        <span class="c1">// more stuff here, from/to whom, stated reason, execution date and such</span>
    <span class="p">}</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">getAmount</span><span class="p">()</span>
    <span class="p">{</span>
        <span class="k">return</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">amount</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="using-dql">
<h2>Using DQL<a class="headerlink" href="#using-dql" title="Permalink to this headline"></a></h2>
<p>The Doctrine Query Language allows you to select for aggregate
values computed from fields of your Domain Model. You can select
the current balance of your account by calling:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="nv">$dql</span> <span class="o">=</span> <span class="s2">&quot;SELECT SUM(e.amount) AS balance FROM Bank\Entities\Entry e &quot;</span> <span class="o">.</span>
       <span class="s2">&quot;WHERE e.account = ?1&quot;</span><span class="p">;</span>
<span class="nv">$balance</span> <span class="o">=</span> <span class="nv">$em</span><span class="o">-&gt;</span><span class="na">createQuery</span><span class="p">(</span><span class="nv">$dql</span><span class="p">)</span>
              <span class="o">-&gt;</span><span class="na">setParameter</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="nv">$myAccountId</span><span class="p">)</span>
              <span class="o">-&gt;</span><span class="na">getSingleScalarResult</span><span class="p">();</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">$em</span></tt> variable in this (and forthcoming) example holds the
Doctrine <tt class="docutils literal"><span class="pre">EntityManager</span></tt>. We create a query for the SUM of all
amounts (negative amounts are withdraws) and retrieve them as a
single scalar result, essentially return only the first column of
the first row.</p>
<p>This approach is simple and powerful, however it has a serious
drawback. We have to execute a specific query for the balance
whenever we need it.</p>
<p>To implement a powerful domain model we would rather have access to
the balance from our <tt class="docutils literal"><span class="pre">Account</span></tt> entity during all times (even if
the Account was not persisted in the database before!).</p>
<p>Also an additional requirement is the max credit per <tt class="docutils literal"><span class="pre">Account</span></tt>
rule.</p>
<p>We cannot reliably enforce this rule in our <tt class="docutils literal"><span class="pre">Account</span></tt> entity with
the DQL retrieval of the balance. There are many different ways to
retrieve accounts. We cannot guarantee that we can execute the
aggregation query for all these use-cases, let alone that a
userland programmer checks this balance against newly added
entries.</p>
</div>
<div class="section" id="using-your-domain-model">
<h2>Using your Domain Model<a class="headerlink" href="#using-your-domain-model" title="Permalink to this headline"></a></h2>
<p><tt class="docutils literal"><span class="pre">Account</span></tt> and all the <tt class="docutils literal"><span class="pre">Entry</span></tt> instances are connected through a
collection, which means we can compute this value at runtime:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">class</span> <span class="nc">Account</span>
<span class="p">{</span>
    <span class="c1">// .. previous code</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">getBalance</span><span class="p">()</span>
    <span class="p">{</span>
        <span class="nv">$balance</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
        <span class="k">foreach</span> <span class="p">(</span><span class="nv">$this</span><span class="o">-&gt;</span><span class="na">entries</span> <span class="k">AS</span> <span class="nv">$entry</span><span class="p">)</span> <span class="p">{</span>
            <span class="nv">$balance</span> <span class="o">+=</span> <span class="nv">$entry</span><span class="o">-&gt;</span><span class="na">getAmount</span><span class="p">();</span>
        <span class="p">}</span>
        <span class="k">return</span> <span class="nv">$balance</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Now we can always call <tt class="docutils literal"><span class="pre">Account::getBalance()</span></tt> to access the
current account balance.</p>
<p>To enforce the max credit rule we have to implement the &#8220;Aggregate
Root&#8221; pattern as described in Eric Evans book on Domain Driven
Design. Described with one sentence, an aggregate root controls the
instance creation, access and manipulation of its children.</p>
<p>In our case we want to enforce that new entries can only added to
the <tt class="docutils literal"><span class="pre">Account</span></tt> by using a designated method. The <tt class="docutils literal"><span class="pre">Account</span></tt> is
the aggregate root of this relation. We can also enforce the
correctness of the bi-directional <tt class="docutils literal"><span class="pre">Account</span></tt> &lt;-&gt; <tt class="docutils literal"><span class="pre">Entry</span></tt>
relation with this method:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">class</span> <span class="nc">Account</span>
<span class="p">{</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">addEntry</span><span class="p">(</span><span class="nv">$amount</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">assertAcceptEntryAllowed</span><span class="p">(</span><span class="nv">$amount</span><span class="p">);</span>

        <span class="nv">$e</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Entry</span><span class="p">(</span><span class="nv">$this</span><span class="p">,</span> <span class="nv">$amount</span><span class="p">);</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">entries</span><span class="p">[]</span> <span class="o">=</span> <span class="nv">$e</span><span class="p">;</span>
        <span class="k">return</span> <span class="nv">$e</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Now look at the following test-code for our entities:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">class</span> <span class="nc">AccountTest</span> <span class="k">extends</span> <span class="nx">\PHPUnit_Framework_TestCase</span>
<span class="p">{</span>
    <span class="k">public</span> <span class="k">function</span> <span class="nf">testAddEntry</span><span class="p">()</span>
    <span class="p">{</span>
        <span class="nv">$account</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Account</span><span class="p">(</span><span class="s2">&quot;123456&quot;</span><span class="p">,</span> <span class="nv">$maxCredit</span> <span class="o">=</span> <span class="mi">200</span><span class="p">);</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">assertEquals</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="nv">$account</span><span class="o">-&gt;</span><span class="na">getBalance</span><span class="p">());</span>

        <span class="nv">$account</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="mi">500</span><span class="p">);</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">assertEquals</span><span class="p">(</span><span class="mi">500</span><span class="p">,</span> <span class="nv">$account</span><span class="o">-&gt;</span><span class="na">getBalance</span><span class="p">());</span>

        <span class="nv">$account</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="o">-</span><span class="mi">700</span><span class="p">);</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">assertEquals</span><span class="p">(</span><span class="o">-</span><span class="mi">200</span><span class="p">,</span> <span class="nv">$account</span><span class="o">-&gt;</span><span class="na">getBalance</span><span class="p">());</span>
    <span class="p">}</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">testExceedMaxLimit</span><span class="p">()</span>
    <span class="p">{</span>
        <span class="nv">$account</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Account</span><span class="p">(</span><span class="s2">&quot;123456&quot;</span><span class="p">,</span> <span class="nv">$maxCredit</span> <span class="o">=</span> <span class="mi">200</span><span class="p">);</span>

        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">setExpectedException</span><span class="p">(</span><span class="s2">&quot;Exception&quot;</span><span class="p">);</span>
        <span class="nv">$account</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="o">-</span><span class="mi">1000</span><span class="p">);</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>To enforce our rule we can now implement the assertion in
<tt class="docutils literal"><span class="pre">Account::addEntry</span></tt>:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">class</span> <span class="nc">Account</span>
<span class="p">{</span>
    <span class="k">private</span> <span class="k">function</span> <span class="nf">assertAcceptEntryAllowed</span><span class="p">(</span><span class="nv">$amount</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="nv">$futureBalance</span> <span class="o">=</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">getBalance</span><span class="p">()</span> <span class="o">+</span> <span class="nv">$amount</span><span class="p">;</span>
        <span class="nv">$allowedMinimalBalance</span> <span class="o">=</span> <span class="p">(</span><span class="nv">$this</span><span class="o">-&gt;</span><span class="na">maxCredit</span> <span class="o">*</span> <span class="o">-</span><span class="mi">1</span><span class="p">);</span>
        <span class="k">if</span> <span class="p">(</span><span class="nv">$futureBalance</span> <span class="o">&lt;</span> <span class="nv">$allowedMinimalBalance</span><span class="p">)</span> <span class="p">{</span>
            <span class="k">throw</span> <span class="k">new</span> <span class="nx">Exception</span><span class="p">(</span><span class="s2">&quot;Credit Limit exceeded, entry is not allowed!&quot;</span><span class="p">);</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>We haven&#8217;t talked to the entity manager for persistence of our
account example before. You can call
<tt class="docutils literal"><span class="pre">EntityManager::persist($account)</span></tt> and then
<tt class="docutils literal"><span class="pre">EntityManager::flush()</span></tt> at any point to save the account to the
database. All the nested <tt class="docutils literal"><span class="pre">Entry</span></tt> objects are automatically
flushed to the database also.</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="nv">$account</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Account</span><span class="p">(</span><span class="s2">&quot;123456&quot;</span><span class="p">,</span> <span class="mi">200</span><span class="p">);</span>
<span class="nv">$account</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="mi">500</span><span class="p">);</span>
<span class="nv">$account</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="o">-</span><span class="mi">200</span><span class="p">);</span>
<span class="nv">$em</span><span class="o">-&gt;</span><span class="na">persist</span><span class="p">(</span><span class="nv">$account</span><span class="p">);</span>
<span class="nv">$em</span><span class="o">-&gt;</span><span class="na">flush</span><span class="p">();</span>
</pre></div>
</div>
<p>The current implementation has a considerable drawback. To get the
balance, we have to initialize the complete <tt class="docutils literal"><span class="pre">Account::$entries</span></tt>
collection, possibly a very large one. This can considerably hurt
the performance of your application.</p>
</div>
<div class="section" id="using-an-aggregate-field">
<h2>Using an Aggregate Field<a class="headerlink" href="#using-an-aggregate-field" title="Permalink to this headline"></a></h2>
<p>To overcome the previously mentioned issue (initializing the whole
entries collection) we want to add an aggregate field called
&#8220;balance&#8221; on the Account and adjust the code in
<tt class="docutils literal"><span class="pre">Account::getBalance()</span></tt> and <tt class="docutils literal"><span class="pre">Account:addEntry()</span></tt>:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">class</span> <span class="nc">Account</span>
<span class="p">{</span>
    <span class="sd">/**</span>
<span class="sd">     * @Column(type=&quot;integer&quot;)</span>
<span class="sd">     */</span>
    <span class="k">private</span> <span class="nv">$balance</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">getBalance</span><span class="p">()</span>
    <span class="p">{</span>
        <span class="k">return</span> <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">balance</span><span class="p">;</span>
    <span class="p">}</span>

    <span class="k">public</span> <span class="k">function</span> <span class="nf">addEntry</span><span class="p">(</span><span class="nv">$amount</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">assertAcceptEntryAllowed</span><span class="p">(</span><span class="nv">$amount</span><span class="p">);</span>

        <span class="nv">$e</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Entry</span><span class="p">(</span><span class="nv">$this</span><span class="p">,</span> <span class="nv">$amount</span><span class="p">);</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">entries</span><span class="p">[]</span> <span class="o">=</span> <span class="nv">$e</span><span class="p">;</span>
        <span class="nv">$this</span><span class="o">-&gt;</span><span class="na">balance</span> <span class="o">+=</span> <span class="nv">$amount</span><span class="p">;</span>
        <span class="k">return</span> <span class="nv">$e</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This is a very simple change, but all the tests still pass. Our
account entities return the correct balance. Now calling the
<tt class="docutils literal"><span class="pre">Account::getBalance()</span></tt> method will not occur the overhead of
loading all entries anymore. Adding a new Entry to the
<tt class="docutils literal"><span class="pre">Account::$entities</span></tt> will also not initialize the collection
internally.</p>
<p>Adding a new entry is therefore very performant and explicitly
hooked into the domain model. It will only update the account with
the current balance and insert the new entry into the database.</p>
</div>
<div class="section" id="tackling-race-conditions-with-aggregate-fields">
<h2>Tackling Race Conditions with Aggregate Fields<a class="headerlink" href="#tackling-race-conditions-with-aggregate-fields" title="Permalink to this headline"></a></h2>
<p>Whenever you denormalize your database schema race-conditions can
potentially lead to inconsistent state. See this example:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="c1">// The Account $accId has a balance of 0 and a max credit limit of 200:</span>
<span class="c1">// request 1 account</span>
<span class="nv">$account1</span> <span class="o">=</span> <span class="nv">$em</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;Bank\Entities\Account&#39;</span><span class="p">,</span> <span class="nv">$accId</span><span class="p">);</span>

<span class="c1">// request 2 account</span>
<span class="nv">$account2</span> <span class="o">=</span> <span class="nv">$em</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;Bank\Entities\Account&#39;</span><span class="p">,</span> <span class="nv">$accId</span><span class="p">);</span>

<span class="nv">$account1</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="o">-</span><span class="mi">200</span><span class="p">);</span>
<span class="nv">$account2</span><span class="o">-&gt;</span><span class="na">addEntry</span><span class="p">(</span><span class="o">-</span><span class="mi">200</span><span class="p">);</span>

<span class="c1">// now request 1 and 2 both flush the changes.</span>
</pre></div>
</div>
<p>The aggregate field <tt class="docutils literal"><span class="pre">Account::$balance</span></tt> is now -200, however the
SUM over all entries amounts yields -400. A violation of our max
credit rule.</p>
<p>You can use both optimistic or pessimistic locking to save-guard
your aggregate fields against this kind of race-conditions. Reading
Eric Evans DDD carefully he mentions that the &#8220;Aggregate Root&#8221;
(Account in our example) needs a locking mechanism.</p>
<p>Optimistic locking is as easy as adding a version column:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">class</span> <span class="nc">Amount</span>
<span class="p">{</span>
    <span class="sd">/** @Column(type=&quot;integer&quot;) @Version */</span>
    <span class="k">private</span> <span class="nv">$version</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The previous example would then throw an exception in the face of
whatever request saves the entity last (and would create the
inconsistent state).</p>
<p>Pessimistic locking requires an additional flag set on the
<tt class="docutils literal"><span class="pre">EntityManager::find()</span></tt> call, enabling write locking directly in
the database using a FOR UPDATE.</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">use</span> <span class="nx">Doctrine\DBAL\LockMode</span><span class="p">;</span>

<span class="nv">$account</span> <span class="o">=</span> <span class="nv">$em</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;Bank\Entities\Account&#39;</span><span class="p">,</span> <span class="nv">$accId</span><span class="p">,</span> <span class="nx">LockMode</span><span class="o">::</span><span class="na">PESSIMISTIC_READ</span><span class="p">);</span>
</pre></div>
</div>
</div>
<div class="section" id="keeping-updates-and-deletes-in-sync">
<h2>Keeping Updates and Deletes in Sync<a class="headerlink" href="#keeping-updates-and-deletes-in-sync" title="Permalink to this headline"></a></h2>
<p>The example shown in this article does not allow changes to the
value in <tt class="docutils literal"><span class="pre">Entry</span></tt>, which considerably simplifies the effort to
keep <tt class="docutils literal"><span class="pre">Account::$balance</span></tt> in sync. If your use-case allows fields
to be updated or related entities to be removed you have to
encapsulate this logic in your &#8220;Aggregate Root&#8221; entity and adjust
the aggregate field accordingly.</p>
</div>
<div class="section" id="conclusion">
<h2>Conclusion<a class="headerlink" href="#conclusion" title="Permalink to this headline"></a></h2>
<p>This article described how to obtain aggregate values using DQL or
your domain model. It showed how you can easily add an aggregate
field that offers serious performance benefits over iterating all
the related objects that make up an aggregate value. Finally I
showed how you can ensure that your aggregate fields do not get out
of sync due to race-conditions and concurrent access.</p>
</div>
</div>


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            <h3><a href="../index.html">Table Of Contents</a></h3>
            <ul>
<li><a class="reference internal" href="#">Aggregate Fields</a><ul>
<li><a class="reference internal" href="#an-example-model">An example model</a></li>
<li><a class="reference internal" href="#using-dql">Using DQL</a></li>
<li><a class="reference internal" href="#using-your-domain-model">Using your Domain Model</a></li>
<li><a class="reference internal" href="#using-an-aggregate-field">Using an Aggregate Field</a></li>
<li><a class="reference internal" href="#tackling-race-conditions-with-aggregate-fields">Tackling Race Conditions with Aggregate Fields</a></li>
<li><a class="reference internal" href="#keeping-updates-and-deletes-in-sync">Keeping Updates and Deletes in Sync</a></li>
<li><a class="reference internal" href="#conclusion">Conclusion</a></li>
</ul>
</li>
</ul>

            <h4>Previous topic</h4>
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                                  title="previous chapter">32. Advanced Configuration</a></p>
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                                  title="next chapter">Persisting the Decorator Pattern</a></p>
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