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<h3><a href="../../../index.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">12. Advanced searching - Django Database queries</a><ul>
<li><a class="reference internal" href="#queryset-field-lookups">12.1. Queryset field lookups</a></li>
<li><a class="reference internal" href="#get-that-werewolf">12.2. Get that werewolf …</a></li>
<li><a class="reference internal" href="#complex-queries">12.3. Complex queries</a></li>
<li><a class="reference internal" href="#annotations">12.4. Annotations</a></li>
<li><a class="reference internal" href="#f-objects">12.5. F-objects</a></li>
<li><a class="reference internal" href="#grouping-and-returning-only-certain-properties">12.6. Grouping and returning only certain properties</a></li>
<li><a class="reference internal" href="#conclusions">12.7. Conclusions</a></li>
</ul>
</li>
</ul>
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<section class="tex2jax_ignore mathjax_ignore" id="advanced-searching-django-database-queries">
<h1><span class="section-number">12. </span>Advanced searching - Django Database queries<a class="headerlink" href="#advanced-searching-django-database-queries" title="Permalink to this headline"></a></h1>
<div class="admonition important">
<p class="admonition-title">Important</p>
<p>More advanced lesson!</p>
<p>Learning about Django’s query language is very useful once you start doing more
advanced things in Evennia. But it’s not strictly needed out the box and can be
a little overwhelming for a first reading. So if you are new to Python and
Evennia, feel free to just skim this lesson and refer back to it later when
you’ve gained more experience.</p>
</div>
<p>The search functions and methods we used in the previous lesson are enough for most cases.
But sometimes you need to be more specific:</p>
<ul class="simple">
<li><p>You want to find all <code class="docutils literal notranslate"><span class="pre">Characters</span></code> …</p></li>
<li><p>… who are in Rooms tagged as <code class="docutils literal notranslate"><span class="pre">moonlit</span></code> …</p></li>
<li><p>… <em>and</em> who has the Attribute <code class="docutils literal notranslate"><span class="pre">lycantrophy</span></code> with a level higher than 2 …</p></li>
<li><p>… because they should immediately transform to werewolves!</p></li>
</ul>
<p>In principle you could achieve this with the existing search functions combined with a lot of loops
and if statements. But for something non-standard like this, querying the database directly will be
much more efficient.</p>
<p>Evennia uses <a class="reference external" href="https://www.djangoproject.com/">Django</a> to handle its connection to the database.
A <a class="reference external" href="https://docs.djangoproject.com/en/3.0/ref/models/querysets/">django queryset</a> represents
a database query. One can add querysets together to build ever-more complicated queries. Only when
you are trying to use the results of the queryset will it actually call the database.</p>
<p>The normal way to build a queryset is to define what class of entity you want to search by getting its
<code class="docutils literal notranslate"><span class="pre">.objects</span></code> resource, and then call various methods on that. We’ve seen this one before:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>all_weapons = Weapon.objects.all()
</pre></div>
</div>
<p>This is now a queryset representing all instances of <code class="docutils literal notranslate"><span class="pre">Weapon</span></code>. If <code class="docutils literal notranslate"><span class="pre">Weapon</span></code> had a subclass <code class="docutils literal notranslate"><span class="pre">Cannon</span></code> and we
only wanted the cannons, we would do</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>all_cannons = Cannon.objects.all()
</pre></div>
</div>
<p>Note that <code class="docutils literal notranslate"><span class="pre">Weapon</span></code> and <code class="docutils literal notranslate"><span class="pre">Cannon</span></code> are <em>different</em> typeclasses. This means that you
won’t find any <code class="docutils literal notranslate"><span class="pre">Weapon</span></code>-typeclassed results in <code class="docutils literal notranslate"><span class="pre">all_cannons</span></code>. Vice-versa, you
won’t find any <code class="docutils literal notranslate"><span class="pre">Cannon</span></code>-typeclassed results in <code class="docutils literal notranslate"><span class="pre">all_weapons</span></code>. This may not be
what you expect.</p>
<p>If you want to get all entities with typeclass <code class="docutils literal notranslate"><span class="pre">Weapon</span></code> <em>as well</em> as all the
subclasses of <code class="docutils literal notranslate"><span class="pre">Weapon</span></code>, such as <code class="docutils literal notranslate"><span class="pre">Cannon</span></code>, you need to use the <code class="docutils literal notranslate"><span class="pre">_family</span></code> type of
query:</p>
<aside class="sidebar">
<p class="sidebar-title">_family</p>
<p>The all_family, filter_family etc is an Evennia-specific
thing. It’s not part of regular Django.</p>
</aside>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>really_all_weapons = Weapon.objects.all_family()
</pre></div>
</div>
<p>This result now contains both <code class="docutils literal notranslate"><span class="pre">Weapon</span></code> and <code class="docutils literal notranslate"><span class="pre">Cannon</span></code> instances (and any other
entities whose typeclasses inherit at any distance from <code class="docutils literal notranslate"><span class="pre">Weapon</span></code>, like <code class="docutils literal notranslate"><span class="pre">Musket</span></code> or
<code class="docutils literal notranslate"><span class="pre">Sword</span></code>).</p>
<p>To limit your search by other criteria than the Typeclass you need to use <code class="docutils literal notranslate"><span class="pre">.filter</span></code>
(or <code class="docutils literal notranslate"><span class="pre">.filter_family</span></code>) instead:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>roses = Flower.objects.filter(db_key=&quot;rose&quot;)
</pre></div>
</div>
<p>This is a queryset representing all flowers having a <code class="docutils literal notranslate"><span class="pre">db_key</span></code> equal to <code class="docutils literal notranslate"><span class="pre">&quot;rose&quot;</span></code>.
Since this is a queryset you can keep adding to it; this will act as an <code class="docutils literal notranslate"><span class="pre">AND</span></code> condition.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>local_roses = roses.filter(db_location=myroom)
</pre></div>
</div>
<p>We could also have written this in one statement:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>local_roses = Flower.objects.filter(db_key=&quot;rose&quot;, db_location=myroom)
</pre></div>
</div>
<p>We can also <code class="docutils literal notranslate"><span class="pre">.exclude</span></code> something from results</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>local_non_red_roses = local_roses.exclude(db_key=&quot;red_rose&quot;)
</pre></div>
</div>
<p>It’s important to note that we haven’t called the database yet! Not until we
actually try to examine the result will the database be called. Here the
database is called when we try to loop over it (because now we need to actually
get results out of it to be able to loop):</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>for rose in local_non_red_roses:
print(rose)
</pre></div>
</div>
<p>From now on, the queryset is <em>evaluated</em> and we can’t keep adding more queries to it - we’d need to
create a new queryset if we wanted to find some other result. Other ways to evaluate the queryset is to
print it, convert it to a list with <code class="docutils literal notranslate"><span class="pre">list()</span></code> and otherwise try to access its results.</p>
<p>Note how we use <code class="docutils literal notranslate"><span class="pre">db_key</span></code> and <code class="docutils literal notranslate"><span class="pre">db_location</span></code>. This is the actual names of these
database fields. By convention Evennia uses <code class="docutils literal notranslate"><span class="pre">db_</span></code> in front of every database
field. When you use the normal Evennia search helpers and objects you can skip
the <code class="docutils literal notranslate"><span class="pre">db_</span></code> but here we are calling the database directly and need to use the
‘real’ names.</p>
<aside class="sidebar">
<p class="sidebar-title">database fields</p>
<p>Each database table have only a few fields. For <code class="docutils literal notranslate"><span class="pre">Objects</span></code>, the most common ones
are <code class="docutils literal notranslate"><span class="pre">db_key</span></code>, <code class="docutils literal notranslate"><span class="pre">db_location</span></code> and <code class="docutils literal notranslate"><span class="pre">db_destination</span></code>. When accessing them they are
normally accessed just as <code class="docutils literal notranslate"><span class="pre">obj.key</span></code>, <code class="docutils literal notranslate"><span class="pre">obj.location</span></code> and <code class="docutils literal notranslate"><span class="pre">obj.destination</span></code>. You
only need to remember the <code class="docutils literal notranslate"><span class="pre">db_</span></code> when using them in database queries. The object
description, <code class="docutils literal notranslate"><span class="pre">obj.db.desc</span></code> is not such a hard-coded field, but one of many
arbitrary Attributes attached to the Object.</p>
</aside>
<p>Here are the most commonly used methods to use with the <code class="docutils literal notranslate"><span class="pre">objects</span></code> managers:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">filter</span></code> - query for a listing of objects based on search criteria. Gives empty queryset if none
were found.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">get</span></code> - query for a single match - raises exception if none were found, or more than one was
found.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">all</span></code> - get all instances of the particular type.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">filter_family</span></code> - like <code class="docutils literal notranslate"><span class="pre">filter</span></code>, but search all sub classes as well.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">get_family</span></code> - like <code class="docutils literal notranslate"><span class="pre">get</span></code>, but search all sub classes as well.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">all_family</span></code> - like <code class="docutils literal notranslate"><span class="pre">all</span></code>, but return entities of all subclasses as well.</p></li>
</ul>
<blockquote>
<div><p>All of Evennia search functions use querysets under the hood. The <code class="docutils literal notranslate"><span class="pre">evennia.search_*</span></code> functions actually
return querysets, which means you could in principle keep adding queries to their results as well.</p>
</div></blockquote>
<section id="queryset-field-lookups">
<h2><span class="section-number">12.1. </span>Queryset field lookups<a class="headerlink" href="#queryset-field-lookups" title="Permalink to this headline"></a></h2>
<p>Above we found roses with exactly the <code class="docutils literal notranslate"><span class="pre">db_key</span></code> <code class="docutils literal notranslate"><span class="pre">&quot;rose&quot;</span></code>. This is an <em>exact</em> match that is <em>case sensitive</em>,
so it would not find <code class="docutils literal notranslate"><span class="pre">&quot;Rose&quot;</span></code>.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span># this is case-sensitive and the same as =
roses = Flower.objects.filter(db_key__exact=&quot;rose&quot;
# the i means it&#39;s case-insensitive
roses = Flower.objects.filter(db_key__iexact=&quot;rose&quot;)
</pre></div>
</div>
<p>The Django field query language uses <code class="docutils literal notranslate"><span class="pre">__</span></code> similarly to how Python uses <code class="docutils literal notranslate"><span class="pre">.</span></code> to access resources. This
is because <code class="docutils literal notranslate"><span class="pre">.</span></code> is not allowed in a function keyword.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>roses = Flower.objects.filter(db_key__icontains=&quot;rose&quot;)
</pre></div>
</div>
<p>This will find all flowers whose name contains the string <code class="docutils literal notranslate"><span class="pre">&quot;rose&quot;</span></code>, like <code class="docutils literal notranslate"><span class="pre">&quot;roses&quot;</span></code>, <code class="docutils literal notranslate"><span class="pre">&quot;wild</span> <span class="pre">rose&quot;</span></code> etc. The
<code class="docutils literal notranslate"><span class="pre">i</span></code> in the beginning makes the search case-insensitive. Other useful variations to use
are <code class="docutils literal notranslate"><span class="pre">__istartswith</span></code> and <code class="docutils literal notranslate"><span class="pre">__iendswith</span></code>. You can also use <code class="docutils literal notranslate"><span class="pre">__gt</span></code>, <code class="docutils literal notranslate"><span class="pre">__ge</span></code> for “greater-than”/“greater-or-equal-than”
comparisons (same for <code class="docutils literal notranslate"><span class="pre">__lt</span></code> and <code class="docutils literal notranslate"><span class="pre">__le</span></code>). There is also <code class="docutils literal notranslate"><span class="pre">__in</span></code>:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>swords = Weapons.objects.filter(db_key__in=(&quot;rapier&quot;, &quot;two-hander&quot;, &quot;shortsword&quot;))
</pre></div>
</div>
<p>One also uses <code class="docutils literal notranslate"><span class="pre">__</span></code> to access foreign objects like Tags. Let’s for example assume
this is how we have identified mages:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>char.tags.add(&quot;mage&quot;, category=&quot;profession&quot;)
</pre></div>
</div>
<p>Now, in this case we have an Evennia helper to do this search:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>mages = evennia.search_tags(&quot;mage&quot;, category=&quot;profession&quot;)
</pre></div>
</div>
<p>But this will find all Objects with this tag+category. Maybe you are only looking for Vampire mages:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>sparkly_mages = Vampire.objects.filter(db_tags__db_key=&quot;mage&quot;, db_tags__db_category=&quot;profession&quot;)
</pre></div>
</div>
<p>This looks at the <code class="docutils literal notranslate"><span class="pre">db_tags</span></code> field on the <code class="docutils literal notranslate"><span class="pre">Vampire</span></code> and filters on the values of each tag’s
<code class="docutils literal notranslate"><span class="pre">db_key</span></code> and <code class="docutils literal notranslate"><span class="pre">db_category</span></code> together.</p>
<p>For more field lookups, see the
<a class="reference external" href="https://docs.djangoproject.com/en/3.0/ref/models/querysets/#field-lookups">django docs</a> on the subject.</p>
</section>
<section id="get-that-werewolf">
<h2><span class="section-number">12.2. </span>Get that werewolf …<a class="headerlink" href="#get-that-werewolf" title="Permalink to this headline"></a></h2>
<p>Let’s see if we can make a query for the werewolves in the moonlight we mentioned at the beginning
of this lesson.</p>
<p>Firstly, we make ourselves and our current location match the criteria, so we can test:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; py here.tags.add(&quot;moonlit&quot;)
&gt; py me.db.lycantrophy = 3
</pre></div>
</div>
<p>This is an example of a more complex query. We’ll consider it an example of what is
possible.</p>
<aside class="sidebar">
<p class="sidebar-title">Line breaks</p>
<p>Note the way of writing this code. It would have been very hard to read if we
just wrote it in one long line. But since we wrapped it in <code class="docutils literal notranslate"><span class="pre">(...)</span></code> we can spread
it out over multiple lines without worrying about line breaks!</p>
</aside>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typeclasses.characters</span> <span class="kn">import</span> <span class="n">Character</span>
<span class="n">will_transform</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">Character</span><span class="o">.</span><span class="n">objects</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span>
<span class="n">db_location__db_tags__db_key__iexact</span><span class="o">=</span><span class="s2">&quot;moonlit&quot;</span><span class="p">,</span>
<span class="n">db_attributes__db_key</span><span class="o">=</span><span class="s2">&quot;lycantrophy&quot;</span><span class="p">,</span>
<span class="n">db_attributes__db_value__gt</span><span class="o">=</span><span class="mi">2</span>
<span class="p">)</span>
<span class="p">)</span>
</pre></div>
</div>
<ul class="simple">
<li><p>We want to find <code class="docutils literal notranslate"><span class="pre">Character</span></code>s, so we access <code class="docutils literal notranslate"><span class="pre">.objects</span></code> on the <code class="docutils literal notranslate"><span class="pre">Character</span></code> typeclass.</p></li>
<li><p>We start to filter …</p></li>
<li><ul>
<li><p>… by accessing the <code class="docutils literal notranslate"><span class="pre">db_location</span></code> field (usually this is a Room)</p></li>
<li><p>… and on that location, we get the value of <code class="docutils literal notranslate"><span class="pre">db_tags</span></code> (this is a <em>many-to-many</em> database field
that we can treat like an object for this purpose; it references all Tags on the location)</p></li>
<li><p>… and from those <code class="docutils literal notranslate"><span class="pre">Tags</span></code>, we looking for <code class="docutils literal notranslate"><span class="pre">Tags</span></code> whose <code class="docutils literal notranslate"><span class="pre">db_key</span></code> is “monlit” (non-case sensitive).</p></li>
</ul>
</li>
<li><p>… We also want only Characters with <code class="docutils literal notranslate"><span class="pre">Attributes</span></code> whose <code class="docutils literal notranslate"><span class="pre">db_key</span></code> is exactly <code class="docutils literal notranslate"><span class="pre">&quot;lycantrophy&quot;</span></code></p></li>
<li><p>… at the same time as the <code class="docutils literal notranslate"><span class="pre">Attribute</span></code>’s <code class="docutils literal notranslate"><span class="pre">db_value</span></code> is greater-than 2.</p></li>
</ul>
<p>Running this query makes our newly lycantrophic Character appear in <code class="docutils literal notranslate"><span class="pre">will_transform</span></code> so we
know to transform it. Success!</p>
<blockquote>
<div><p>Don’t confuse database fields with <a class="reference internal" href="../../../Components/Attributes.html"><span class="doc std std-doc">Attributes</span></a> you set via <code class="docutils literal notranslate"><span class="pre">obj.db.attr</span> <span class="pre">=</span> <span class="pre">'foo'</span></code> or
<code class="docutils literal notranslate"><span class="pre">obj.attributes.add()</span></code>. Attributes are custom database entities <em>linked</em> to an object. They are not
separate fields <em>on</em> that object like <code class="docutils literal notranslate"><span class="pre">db_key</span></code> or <code class="docutils literal notranslate"><span class="pre">db_location</span></code> are.</p>
</div></blockquote>
</section>
<section id="complex-queries">
<h2><span class="section-number">12.3. </span>Complex queries<a class="headerlink" href="#complex-queries" title="Permalink to this headline"></a></h2>
<p>All examples so far used <code class="docutils literal notranslate"><span class="pre">AND</span></code> relations. The arguments to <code class="docutils literal notranslate"><span class="pre">.filter</span></code> are added together with <code class="docutils literal notranslate"><span class="pre">AND</span></code>
(“we want tag room to be “monlit” <em>and</em> lycantrhopy be &gt; 2”).</p>
<p>For queries using <code class="docutils literal notranslate"><span class="pre">OR</span></code> and <code class="docutils literal notranslate"><span class="pre">NOT</span></code> we need Django’s
<a class="reference external" href="https://docs.djangoproject.com/en/1.11/topics/db/queries/#complex-lookups-with-q-objects">Q object</a>. It is
imported from Django directly:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>from django.db.models import Q
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">Q</span></code> is an object that is created with the same arguments as <code class="docutils literal notranslate"><span class="pre">.filter</span></code>, for example</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Q(db_key=&quot;foo&quot;)
</pre></div>
</div>
<p>You can then use this <code class="docutils literal notranslate"><span class="pre">Q</span></code> instance as argument in a <code class="docutils literal notranslate"><span class="pre">filter</span></code>:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>q1 = Q(db_key=&quot;foo&quot;)
Character.objects.filter(q1)
</pre></div>
</div>
<p>The useful thing about <code class="docutils literal notranslate"><span class="pre">Q</span></code> is that these objects can be chained together with special symbols (bit operators):
<code class="docutils literal notranslate"><span class="pre">|</span></code> for <code class="docutils literal notranslate"><span class="pre">OR</span></code> and <code class="docutils literal notranslate"><span class="pre">&amp;</span></code> for <code class="docutils literal notranslate"><span class="pre">AND</span></code>. A tilde <code class="docutils literal notranslate"><span class="pre">~</span></code> in front negates the expression inside the <code class="docutils literal notranslate"><span class="pre">Q</span></code> and thus
works like <code class="docutils literal notranslate"><span class="pre">NOT</span></code>.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>q1 = Q(db_key=&quot;Dalton&quot;)
q2 = Q(db_location=prison)
Character.objects.filter(q1 | ~q2)
</pre></div>
</div>
<p>Would get all Characters that are either named “Dalton” <em>or</em> which is <em>not</em> in prison. The result is a mix
of Daltons and non-prisoners.</p>
<p>Let us expand our original werewolf query. Not only do we want to find all
Characters in a moonlit room with a certain level of <code class="docutils literal notranslate"><span class="pre">lycanthrophy</span></code>. Now we also
want the full moon to immediately transform people who were recently bitten,
even if their <code class="docutils literal notranslate"><span class="pre">lycantrophy</span></code> level is not yet high enough (more dramatic this
way!). When you get bitten, you’ll get a Tag <code class="docutils literal notranslate"><span class="pre">recently_bitten</span></code> put on you to
indicate this.</p>
<p>This is how we’d change our query:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="kn">import</span> <span class="n">Q</span>
<span class="n">will_transform</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">Character</span><span class="o">.</span><span class="n">objects</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span>
<span class="n">Q</span><span class="p">(</span><span class="n">db_location__db_tags__db_key__iexact</span><span class="o">=</span><span class="s2">&quot;moonlit&quot;</span><span class="p">)</span>
<span class="o">&amp;</span> <span class="p">(</span>
<span class="n">Q</span><span class="p">(</span><span class="n">db_attributes__db_key</span><span class="o">=</span><span class="s2">&quot;lycantrophy&quot;</span><span class="p">,</span>
<span class="n">db_attributes__db_value__gt</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="o">|</span> <span class="n">Q</span><span class="p">(</span><span class="n">db_tags__db_key__iexact</span><span class="o">=</span><span class="s2">&quot;recently_bitten&quot;</span><span class="p">)</span>
<span class="p">))</span>
<span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="p">)</span>
</pre></div>
</div>
<p>That’s quite compact. It may be easier to see what’s going on if written this way:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="kn">import</span> <span class="n">Q</span>
<span class="n">q_moonlit</span> <span class="o">=</span> <span class="n">Q</span><span class="p">(</span><span class="n">db_location__db_tags__db_key__iexact</span><span class="o">=</span><span class="s2">&quot;moonlit&quot;</span><span class="p">)</span>
<span class="n">q_lycantropic</span> <span class="o">=</span> <span class="n">Q</span><span class="p">(</span><span class="n">db_attributes__db_key</span><span class="o">=</span><span class="s2">&quot;lycantrophy&quot;</span><span class="p">,</span> <span class="n">db_attributes__db_value__gt</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="n">q_recently_bitten</span> <span class="o">=</span> <span class="n">Q</span><span class="p">(</span><span class="n">db_tags__db_key__iexact</span><span class="o">=</span><span class="s2">&quot;recently_bitten&quot;</span><span class="p">)</span>
<span class="n">will_transform</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">Character</span><span class="o">.</span><span class="n">objects</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">q_moonlit</span> <span class="o">&amp;</span> <span class="p">(</span><span class="n">q_lycantropic</span> <span class="o">|</span> <span class="n">q_recently_bitten</span><span class="p">))</span>
<span class="o">.</span><span class="n">distinct</span><span class="p">()</span>
<span class="p">)</span>
</pre></div>
</div>
<aside class="sidebar">
<p class="sidebar-title">SQL</p>
<p>These Python structures are internally converted to SQL, the native language of
the database. If you are familiar with SQL, these are many-to-many tables
joined with <code class="docutils literal notranslate"><span class="pre">LEFT</span> <span class="pre">OUTER</span> <span class="pre">JOIN</span></code>, which may lead to multiple merged rows combining
the same object with different relations.</p>
</aside>
<p>This reads as “Find all Characters in a moonlit room that either has the
Attribute <code class="docutils literal notranslate"><span class="pre">lycantrophy</span></code> higher than two, <em>or</em> which has the Tag
<code class="docutils literal notranslate"><span class="pre">recently_bitten</span></code>”. With an OR-query like this it’s possible to find the same
Character via different paths, so we add <code class="docutils literal notranslate"><span class="pre">.distinct()</span></code> at the end. This makes
sure that there is only one instance of each Character in the result.</p>
</section>
<section id="annotations">
<h2><span class="section-number">12.4. </span>Annotations<a class="headerlink" href="#annotations" title="Permalink to this headline"></a></h2>
<p>What if we wanted to filter on some condition that isn’t represented easily by a
field on the object? Maybe we want to find rooms only containing five or more
objects?</p>
<p>We <em>could</em> do it like this (don’t actually do it this way!):</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="kn">from</span> <span class="nn">typeclasses.rooms</span> <span class="kn">import</span> <span class="n">Room</span>
<span class="n">all_rooms</span> <span class="o">=</span> <span class="n">Rooms</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">all</span><span class="p">()</span>
<span class="n">rooms_with_five_objects</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">room</span> <span class="ow">in</span> <span class="n">all_rooms</span><span class="p">:</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">room</span><span class="o">.</span><span class="n">contents</span><span class="p">)</span> <span class="o">&gt;=</span> <span class="mi">5</span><span class="p">:</span>
<span class="n">rooms_with_five_objects</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">room</span><span class="p">)</span>
</pre></div>
</div>
<p>Above we get all rooms and then use <code class="docutils literal notranslate"><span class="pre">list.append()</span></code> to keep adding the right
rooms to an ever-growing list. This is <em>not</em> a good idea, once your database
grows this will be unnecessarily computing-intensive. The database is much more
suitable for this.</p>
<p><em>Annotations</em> allow you to set a ‘variable’ inside the query that you can then
access from other parts of the query. Let’s do the same example as before
directly in the database:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typeclasses.rooms</span> <span class="kn">import</span> <span class="n">Room</span>
<span class="kn">from</span> <span class="nn">django.db.models</span> <span class="kn">import</span> <span class="n">Count</span>
<span class="n">rooms</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">Room</span><span class="o">.</span><span class="n">objects</span>
<span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="n">num_objects</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s1">&#39;locations_set&#39;</span><span class="p">))</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">num_objects__gte</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span>
<span class="p">)</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">Count</span></code> is a Django class for counting the number of things in the database.</p>
<p>Here we first create an annotation <code class="docutils literal notranslate"><span class="pre">num_objects</span></code> of type <code class="docutils literal notranslate"><span class="pre">Count</span></code>. It creates an in-database function
that will count the number of results inside the database.</p>
<blockquote>
<div><p>Note the use of <code class="docutils literal notranslate"><span class="pre">location_set</span></code> in that <code class="docutils literal notranslate"><span class="pre">Count</span></code>. The <code class="docutils literal notranslate"><span class="pre">*_set</span></code> is a back-reference automatically created by
Django. In this case it allows you to find all objects that <em>has the current object as location</em>.</p>
</div></blockquote>
<p>Next we filter on this annotation, using the name <code class="docutils literal notranslate"><span class="pre">num_objects</span></code> as something we
can filter for. We use <code class="docutils literal notranslate"><span class="pre">num_objects__gte=5</span></code> which means that <code class="docutils literal notranslate"><span class="pre">num_objects</span></code>
should be greater than or equal to 5. This is a little harder to get one’s head
around but much more efficient than lopping over all objects in Python.</p>
</section>
<section id="f-objects">
<h2><span class="section-number">12.5. </span>F-objects<a class="headerlink" href="#f-objects" title="Permalink to this headline"></a></h2>
<p>What if we wanted to compare two dynamic parameters against one another in a
query? For example, what if instead of having 5 or more objects, we only wanted
objects that had a bigger inventory than they had tags (silly example, but …)?
This can be with Django’s <a class="reference external" href="https://docs.djangoproject.com/en/1.11/ref/models/expressions/#f-expressions">F objects</a>.
So-called F expressions allow you to do a query that looks at a value of each
object in the database.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="kn">import</span> <span class="n">Count</span><span class="p">,</span> <span class="n">F</span>
<span class="kn">from</span> <span class="nn">typeclasses.rooms</span> <span class="kn">import</span> <span class="n">Room</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">Room</span><span class="o">.</span><span class="n">objects</span>
<span class="o">.</span><span class="n">annotate</span><span class="p">(</span>
<span class="n">num_objects</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s1">&#39;locations_set&#39;</span><span class="p">),</span>
<span class="n">num_tags</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s1">&#39;db_tags&#39;</span><span class="p">))</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">num_objects__gt</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">&#39;num_tags&#39;</span><span class="p">))</span>
<span class="p">)</span>
</pre></div>
</div>
<p>Here we used <code class="docutils literal notranslate"><span class="pre">.annotate</span></code> to create two in-query ‘variables’ <code class="docutils literal notranslate"><span class="pre">num_objects</span></code> and <code class="docutils literal notranslate"><span class="pre">num_tags</span></code>. We then
directly use these results in the filter. Using <code class="docutils literal notranslate"><span class="pre">F()</span></code> allows for also the right-hand-side of the filter
condition to be calculated on the fly, completely within the database.</p>
</section>
<section id="grouping-and-returning-only-certain-properties">
<h2><span class="section-number">12.6. </span>Grouping and returning only certain properties<a class="headerlink" href="#grouping-and-returning-only-certain-properties" title="Permalink to this headline"></a></h2>
<p>Suppose you used tags to mark someone belonging to an organization. Now you want to make a list and
need to get the membership count of every organization all at once.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">.annotate</span></code>, <code class="docutils literal notranslate"><span class="pre">.values_list</span></code>, and <code class="docutils literal notranslate"><span class="pre">.order_by</span></code> queryset methods are useful for this. Normally when
you run a <code class="docutils literal notranslate"><span class="pre">.filter</span></code>, what you get back is a bunch of full typeclass instances, like roses or swords.
Using <code class="docutils literal notranslate"><span class="pre">.values_list</span></code> you can instead choose to only get back certain properties on objects.
The <code class="docutils literal notranslate"><span class="pre">.order_by</span></code> method finally allows for sorting the results according to some criterion:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="kn">import</span> <span class="n">Count</span>
<span class="kn">from</span> <span class="nn">typeclasses.rooms</span> <span class="kn">import</span> <span class="n">Room</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">Character</span><span class="o">.</span><span class="n">objects</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">db_tags__db_category</span><span class="o">=</span><span class="s2">&quot;organization&quot;</span><span class="p">)</span>
<span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">tagcount</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s1">&#39;id&#39;</span><span class="p">))</span>
<span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s1">&#39;-tagcount&#39;</span><span class="p">))</span>
<span class="o">.</span><span class="n">values_list</span><span class="p">(</span><span class="s1">&#39;db_tags__db_key&#39;</span><span class="p">,</span> <span class="s2">&quot;tagcount&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>Here we fetch all Characters who …</p>
<ul class="simple">
<li><p>… has a tag of category “organization” on them</p></li>
<li><p>… along the way we count how many different Characters (each <code class="docutils literal notranslate"><span class="pre">id</span></code> is unique) we find for each organization
and store it in a ‘variable’ <code class="docutils literal notranslate"><span class="pre">tagcount</span></code> using <code class="docutils literal notranslate"><span class="pre">.annotate</span></code> and <code class="docutils literal notranslate"><span class="pre">Count</span></code></p></li>
<li><p>… we use this count to sort the result in descending order of <code class="docutils literal notranslate"><span class="pre">tagcount</span></code> (descending because there is a minus sign,
default is increasing order but we want the most popular organization to be first).</p></li>
<li><p>… and finally we make sure to only return exactly the properties we want, namely the name of the organization tag
and how many matches we found for that organization.</p></li>
</ul>
<p>The result queryset will be a list of tuples ordered in descending order by the number of matches,
in a format like the following:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span>
<span class="p">(</span><span class="s1">&#39;Griatch&#39;</span><span class="n">s</span> <span class="n">poets</span> <span class="n">society</span><span class="s1">&#39;, 3872),</span>
<span class="p">(</span><span class="s2">&quot;Chainsol&#39;s Ainneve Testers&quot;</span><span class="p">,</span> <span class="mi">2076</span><span class="p">),</span>
<span class="p">(</span><span class="s2">&quot;Blaufeuer&#39;s Whitespace Fixers&quot;</span><span class="p">,</span> <span class="mi">1903</span><span class="p">),</span>
<span class="p">(</span><span class="s2">&quot;Volund&#39;s Bikeshed Design Crew&quot;</span><span class="p">,</span> <span class="mi">1764</span><span class="p">),</span>
<span class="p">(</span><span class="s2">&quot;Tehom&#39;s Glorious Misanthropes&quot;</span><span class="p">,</span> <span class="mi">1763</span><span class="p">)</span>
<span class="p">]</span>
</pre></div>
</div>
</section>
<section id="conclusions">
<h2><span class="section-number">12.7. </span>Conclusions<a class="headerlink" href="#conclusions" title="Permalink to this headline"></a></h2>
<p>We have covered a lot of ground in this lesson and covered several more complex
topics. Knowing how to query using Django is a powerful skill to have.</p>
<p>This concludes the first part of the Evennia starting tutorial - “What we have”.
Now we have a good foundation to understand how to plan what our tutorial game
will be about.</p>
</section>
</section>
</div>
</div>
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