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<h2>FetchGroup</h2>
<p>
As an alternative to <em>select</em> and <em>fetch</em> we can use <em>FetchGroup</em> to specify what part of
the object graph to fetch.
</p>
<p>
We can use <em>FetchGroup</em> with both query beans and standard queries.
</p>
<p>
FetchGroups provide a clean way to separate the definition of "what part of the object graph to load" (query tuning)
from the definition of the query predicates (query business logic).
</p>
<p>
A FetchGroup is immutable, most often declared as a <code>static final</code> and can be combined to make other
FetchGroups.
</p>
<h4>e.g. Just top level select properties</h4>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="c1">// immutable and threadsafe</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">QCustomer</span> <span class="n">CUST</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">FetchGroup</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">fetch</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">forFetchGroup</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">CUST</span><span class="o">.</span><span class="na">name</span><span class="o">,</span> <span class="n">CUST</span><span class="o">.</span><span class="na">version</span><span class="o">,</span> <span class="n">CUST</span><span class="o">.</span><span class="na">whenCreated</span><span class="o">)</span>
<span class="o">.</span><span class="na">buildFetchGroup</span><span class="o">();</span>
<span class="o">...</span>
<span class="n">List</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">customers</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QCustomer</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">fetch</span><span class="o">)</span>
<span class="o">.</span><span class="na field">name</span><span class="o">.</span><span class="na">istartsWith</span><span class="o">(</span><span class="s">"Rob"</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<p>
The above FetchGroup can be created without using query beans like the below:
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="kd">static</span> <span class="kd">final</span> <span class="n">FetchGroup</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">fetch</span> <span class="o">=</span>
<span class="n">FetchGroup</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="n">Customer</span><span class="o">.</span><span class="na">class</span><span class="o">,</span> <span class="s">"name, version, whenCreated"</span><span class="o">);</span> <span class="c1">// root level properties</span>
</pre></div>
</div>
<p> </p>
<h4>e.g. select and fetch properties</h4>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="c1">// immutable and threadsafe</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">QCustomer</span> <span class="n">CUST</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">QContact</span> <span class="n">CONT</span> <span class="o">=</span> <span class="n">QContact</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">FetchGroup</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">fetch</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">forFetchGroup</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">CUST</span><span class="o">.</span><span class="na">name</span><span class="o">,</span> <span class="n">CUST</span><span class="o">.</span><span class="na">version</span><span class="o">,</span> <span class="n">CUST</span><span class="o">,</span><span class="n">whenCreated</span><span class="o">)</span>
<span class="o">.</span><span class="na field">contacts</span><span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="n">CONT</span><span class="o">.</span><span class="na">email</span><span class="o">)</span> <span class="c1">// fetch the contacts with just their email</span>
<span class="o">.</span><span class="na field">billingAddress</span><span class="o">.</span><span class="na">fetch</span><span class="o">()</span> <span class="c1">// fetch all properties of billingAddress</span>
<span class="o">.</span><span class="na">buildFetchGroup</span><span class="o">();</span>
<span class="o">...</span>
<span class="n">List</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">customers</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QCustomer</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">fetch</span><span class="o">)</span>
<span class="o">.</span><span class="na field">name</span><span class="o">.</span><span class="na">istartsWith</span><span class="o">(</span><span class="s">"Rob"</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<p>
The above FetchGroup can be created without query beans by:
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="c1">// immutable and threadsafe</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">FetchGroup</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">fetch</span> <span class="o">=</span>
<span class="n">FetchGroup</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="n">Customer</span><span class="o">.</span><span class="na">class</span><span class="o">)</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="s">"name, status"</span><span class="o">)</span> <span class="c1">// root level properties</span>
<span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="s">"contacts"</span><span class="o">,</span> <span class="s">"email"</span><span class="o">)</span> <span class="c1">// associated bean properties</span>
<span class="o">.</span><span class="na">build</span><span class="o">();</span>
</pre></div>
</div>
<h3>Example</h3>
<p>
This is a bigger example fetching orders with related lines, shipments, customer,
customer.billingAddress and customer.contacts.
</p>
<p>
Note we control which properties (or all) are fetched on each path.
</p>
<p>
In this example we have several ToMany paths and we are using <em>fetchQuery</em> to
explicitly control how the ORM query is broken into multiple SQL queries to build the graph.
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="c1">// immutable and threadsafe</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">QOrder</span> <span class="n">ORD</span> <span class="o">=</span> <span class="n">QOrder</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">QCustomer</span> <span class="n">CUST</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">QContact</span> <span class="n">CONT</span> <span class="o">=</span> <span class="n">QContact</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="kd">static</span> <span class="kd">final</span> <span class="n">FetchGroup</span><span class="o"><</span><span class="n">Order</span><span class="o">></span> <span class="n">fetch</span> <span class="o">=</span> <span class="n">QOrder</span><span class="o">.</span><span class="na">forFetchGroup</span><span class="o">()</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="n">CUST</span><span class="o">.</span><span class="na">name</span><span class="o">,</span> <span class="n">CUST</span><span class="o">.</span><span class="na">version</span><span class="o">,</span> <span class="n">CUST</span><span class="o">,</span><span class="n">whenCreated</span><span class="o">)</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na field">shippingAddress</span><span class="o">.</span><span class="na">fetch</span><span class="o">()</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na field">contacts</span><span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="n">CONT</span><span class="o">.</span><span class="na">email</span><span class="o">)</span> <span class="c1">// a ToMany path</span>
<span class="o">.</span><span class="na field">lines</span><span class="o">.</span><span class="na">fetchQuery</span><span class="o">()</span> <span class="c1">// a ToMany path</span>
<span class="o">.</span><span class="na field">shipments</span><span class="o">.</span><span class="na">fetchQuery</span><span class="o">()</span> <span class="c1">// a ToMany path</span>
<span class="o">.</span><span class="na">buildFetchGroup</span><span class="o">();</span>
<span class="o">...</span>
<span class="n">List</span><span class="o"><</span><span class="n">Order</span><span class="o">></span> <span class="n">orders</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QOrder</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">fetch</span><span class="o">)</span>
<span class="o">.</span><span class="na field">status</span><span class="o">.</span><span class="na">eq</span><span class="o">(</span><span class="n">Order</span><span class="o">.</span><span class="na field">Status</span><span class="o">.</span><span class="na">NEW</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
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