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<div class="body" >
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<div class="section" id="priming-references">
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<h1>Priming References<a class="headerlink" href="#priming-references" title="Permalink to this headline">¶</a></h1>
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<p>Priming references allows you to consolidate database queries when working with
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<a class="reference internal" href="reference-mapping.html#reference-one"><span class="std std-ref">one</span></a> and <a class="reference internal" href="reference-mapping.html#reference-many"><span class="std std-ref">many</span></a> reference mappings.
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This is useful for avoiding the
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<a class="reference external" href="http://stackoverflow.com/q/97197/162228">n+1 problem</a> in your application.</p>
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<div class="section" id="query-builder">
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<h2>Query Builder<a class="headerlink" href="#query-builder" title="Permalink to this headline">¶</a></h2>
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<p>Consider the following abbreviated model:</p>
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<div class="highlight-php"><div class="highlight"><pre><span class="cp"><?php</span>
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<span class="sd">/** @Document */</span>
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<span class="k">class</span> <span class="nc">User</span>
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<span class="p">{</span>
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<span class="sd">/** @ReferenceMany(targetDocument="Account") */</span>
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<span class="k">private</span> <span class="nv">$accounts</span><span class="p">;</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>We would like to query for 100 users and then iterate over their referenced
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accounts.</p>
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<div class="highlight-php"><div class="highlight"><pre><span class="cp"><?php</span>
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<span class="nv">$qb</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-></span><span class="na">createQueryBuilder</span><span class="p">(</span><span class="s1">'User'</span><span class="p">)</span>
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<span class="o">-></span><span class="na">limit</span><span class="p">(</span><span class="mi">100</span><span class="p">);</span>
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<span class="nv">$query</span> <span class="o">=</span> <span class="nv">$qb</span><span class="o">-></span><span class="na">getQuery</span><span class="p">();</span>
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<span class="nv">$users</span> <span class="o">=</span> <span class="nv">$query</span><span class="o">-></span><span class="na">execute</span><span class="p">();</span>
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<span class="k">foreach</span> <span class="p">(</span><span class="nv">$users</span> <span class="k">as</span> <span class="nv">$user</span><span class="p">)</span> <span class="p">{</span>
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<span class="cm">/* PersistentCollection::initialize() will be invoked when we begin</span>
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<span class="cm"> * iterating through the user's accounts. Any accounts not already</span>
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<span class="cm"> * managed by the unit of work will need to be queried.</span>
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<span class="cm"> */</span>
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<span class="k">foreach</span> <span class="p">(</span><span class="nv">$user</span><span class="o">-></span><span class="na">getAccounts</span><span class="p">()</span> <span class="k">as</span> <span class="nv">$account</span><span class="p">)</span> <span class="p">{</span>
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<span class="c1">// ...</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>In this example, ODM would query the database once for the result set of users
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and then, for each user, issue a separate query to load any accounts that are
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not already being managed by the unit of work. This could result in as many as
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100 additional database queries!</p>
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<p>If we expect to iterate through all users and their accounts, we could optimize
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this process by loading all of the referenced accounts with one query. The query
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builder's <code class="docutils literal"><span class="pre">prime()</span></code> method allows us to do just that.</p>
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<div class="highlight-php"><div class="highlight"><pre><span class="cp"><?php</span>
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<span class="nv">$qb</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-></span><span class="na">createQueryBuilder</span><span class="p">(</span><span class="s1">'User'</span><span class="p">)</span>
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<span class="o">-></span><span class="na">field</span><span class="p">(</span><span class="s1">'accounts'</span><span class="p">)</span><span class="o">-></span><span class="na">prime</span><span class="p">(</span><span class="k">true</span><span class="p">)</span>
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<span class="o">-></span><span class="na">limit</span><span class="p">(</span><span class="mi">100</span><span class="p">);</span>
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<span class="nv">$query</span> <span class="o">=</span> <span class="nv">$qb</span><span class="o">-></span><span class="na">getQuery</span><span class="p">();</span>
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<span class="cm">/* After querying for the users, ODM will collect the IDs of all referenced</span>
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<span class="cm"> * accounts and load them with a single additional query.</span>
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<span class="cm"> */</span>
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<span class="nv">$users</span> <span class="o">=</span> <span class="nv">$query</span><span class="o">-></span><span class="na">execute</span><span class="p">();</span>
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<span class="k">foreach</span> <span class="p">(</span><span class="nv">$users</span> <span class="k">as</span> <span class="nv">$user</span><span class="p">)</span> <span class="p">{</span>
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<span class="cm">/* Accounts have already been loaded, so iterating through accounts will</span>
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<span class="cm"> * not query an additional query.</span>
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<span class="cm"> */</span>
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<span class="k">foreach</span> <span class="p">(</span><span class="nv">$user</span><span class="o">-></span><span class="na">getAccounts</span><span class="p">()</span> <span class="k">as</span> <span class="nv">$account</span><span class="p">)</span> <span class="p">{</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>In this case, priming will allow us to load all users and referenced accounts in
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two database queries. If the accounts had used an
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<a class="reference internal" href="inheritance-mapping.html#inheritance-mapping"><span class="std std-ref">inheritance mapping</span></a>, priming might require several
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queries (one per discriminated class name).</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">Priming is also compatible with <a class="reference internal" href="reference-mapping.html#storing-references"><span class="std std-ref">simple references</span></a>
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and discriminated references. When priming discriminated references, ODM
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will issue one query per distinct class among the referenced document(s).</p>
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</div>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">Hydration must be enabled in the query builder for priming to work properly.
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Disabling hydration will cause the DBRef to be returned for a referenced
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document instead of the hydrated document object.</p>
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</div>
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</div>
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<div class="section" id="inverse-references">
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<h2>Inverse references<a class="headerlink" href="#inverse-references" title="Permalink to this headline">¶</a></h2>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">This feature was added in version 1.2.</p>
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</div>
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<p>When using inverse references (references mapped using <code class="docutils literal"><span class="pre">mappedBy</span></code> or
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<code class="docutils literal"><span class="pre">repositoryMethod</span></code>) you can also enable primers on one-to-many references by
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specifying them in the mapping:</p>
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<div class="highlight-php"><div class="highlight"><pre><span class="cp"><?php</span>
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<span class="sd">/** @Document */</span>
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<span class="k">class</span> <span class="nc">User</span>
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<span class="p">{</span>
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<span class="sd">/** @ReferenceMany(targetDocument="Account", prime={"user"}) */</span>
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<span class="k">private</span> <span class="nv">$accounts</span><span class="p">;</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>When the collection is initialized, the configured primers are automatically
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added to the query.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">When using inverse references with <code class="docutils literal"><span class="pre">repositoryMethod</span></code>, be sure to return
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an eager cursor from the repository method if you want to rely on primers
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defined in the mapping. If the result is not an eager cursor, an exception
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will be thrown and the collection won't be loaded. Also, any primers you
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might have added in the <code class="docutils literal"><span class="pre">repositoryMethod</span></code> are overwritten with those
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specified in the mapping.</p>
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</div>
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</div>
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<div class="section" id="primer-callback">
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<h2>Primer Callback<a class="headerlink" href="#primer-callback" title="Permalink to this headline">¶</a></h2>
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<p>Passing <code class="docutils literal"><span class="pre">true</span></code> to <code class="docutils literal"><span class="pre">prime()</span></code> instructs ODM to load the referenced document(s)
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on its own; however, we can also pass a custom callable (e.g. Closure instance)
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to <code class="docutils literal"><span class="pre">prime()</span></code>, which allows more control over the priming query.</p>
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<p>As an example, we can look at the default callable, which is found in the
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<code class="docutils literal"><span class="pre">ReferencePrimer</span></code> class.</p>
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<div class="highlight-php"><div class="highlight"><pre><span class="cp"><?php</span>
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<span class="k">function</span><span class="p">(</span><span class="nx">DocumentManager</span> <span class="nv">$dm</span><span class="p">,</span> <span class="nx">ClassMetadata</span> <span class="nv">$class</span><span class="p">,</span> <span class="k">array</span> <span class="nv">$ids</span><span class="p">,</span> <span class="k">array</span> <span class="nv">$hints</span><span class="p">)</span> <span class="p">{</span>
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<span class="nv">$qb</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-></span><span class="na">createQueryBuilder</span><span class="p">(</span><span class="nv">$class</span><span class="o">-></span><span class="na">name</span><span class="p">)</span>
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<span class="o">-></span><span class="na">field</span><span class="p">(</span><span class="nv">$class</span><span class="o">-></span><span class="na">identifier</span><span class="p">)</span><span class="o">-></span><span class="na">in</span><span class="p">(</span><span class="nv">$ids</span><span class="p">);</span>
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<span class="k">if</span> <span class="p">(</span> <span class="o">!</span> <span class="k">empty</span><span class="p">(</span><span class="nv">$hints</span><span class="p">[</span><span class="nx">Query</span><span class="o">::</span><span class="na">HINT_SLAVE_OKAY</span><span class="p">]))</span> <span class="p">{</span>
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<span class="nv">$qb</span><span class="o">-></span><span class="na">slaveOkay</span><span class="p">(</span><span class="k">true</span><span class="p">);</span>
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<span class="p">}</span>
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<span class="k">if</span> <span class="p">(</span> <span class="o">!</span> <span class="k">empty</span><span class="p">(</span><span class="nv">$hints</span><span class="p">[</span><span class="nx">Query</span><span class="o">::</span><span class="na">HINT_READ_PREFERENCE</span><span class="p">]))</span> <span class="p">{</span>
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<span class="nv">$qb</span><span class="o">-></span><span class="na">setReadPreference</span><span class="p">(</span>
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<span class="nv">$hints</span><span class="p">[</span><span class="nx">Query</span><span class="o">::</span><span class="na">HINT_READ_PREFERENCE</span><span class="p">],</span>
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<span class="nv">$hints</span><span class="p">[</span><span class="nx">Query</span><span class="o">::</span><span class="na">HINT_READ_PREFERENCE_TAGS</span><span class="p">]</span>
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<span class="p">);</span>
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<span class="p">}</span>
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<span class="nv">$qb</span><span class="o">-></span><span class="na">getQuery</span><span class="p">()</span><span class="o">-></span><span class="na">toArray</span><span class="p">();</span>
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<span class="p">};</span>
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</pre></div>
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</div>
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<p>Firstly, the callable is passed the <code class="docutils literal"><span class="pre">DocumentManager</span></code> of the main query. This
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is necessary to create the query used for priming, and ensures that the results
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will become managed in the same scope. The <code class="docutils literal"><span class="pre">ClassMetadata</span></code> argument provides
|
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mapping information for the referenced class as well as its name, which is used
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to create the query builder. An array of identifiers follows, which is used to
|
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query for the documents to be primed. Lastly, the <code class="docutils literal"><span class="pre">UnitOfWork</span></code> hints from the
|
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original query are provided so that the priming query can apply them as well.</p>
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</div>
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</div>
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</div>
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</div>
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</div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">Priming References</a><ul>
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© Copyright 2013, Doctrine Project Team.
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