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@ -54,12 +54,11 @@
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<div class="admonition important">
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<p class="admonition-title">Important</p>
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<p>More advanced lesson!</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Learning about Django's queryset language is very useful once you start doing more advanced things
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in Evennia. But it's not strictly needed out the box and can be a little overwhelming for a first
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reading. So if you are new to Python and Evennia, feel free to just skim this lesson and refer
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back to it later when you've gained more experience.
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</pre></div>
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</div>
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<p>Learning about Django’s query language is very useful once you start doing more
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advanced things in Evennia. But it’s not strictly needed out the box and can be
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a little overwhelming for a first reading. So if you are new to Python and
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Evennia, feel free to just skim this lesson and refer back to it later when
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you’ve gained more experience.</p>
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</div>
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<p>The search functions and methods we used in the previous lesson are enough for most cases.
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But sometimes you need to be more specific:</p>
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@ -67,7 +66,7 @@ But sometimes you need to be more specific:</p>
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<li><p>You want to find all <code class="docutils literal notranslate"><span class="pre">Characters</span></code> …</p></li>
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<li><p>… who are in Rooms tagged as <code class="docutils literal notranslate"><span class="pre">moonlit</span></code> …</p></li>
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<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>
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<li><p>… because they’ll should immediately transform to werewolves!</p></li>
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<li><p>… because they should immediately transform to werewolves!</p></li>
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</ul>
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<p>In principle you could achieve this with the existing search functions combined with a lot of loops
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and if statements. But for something non-standard like this, querying the database directly will be
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@ -86,26 +85,30 @@ only wanted the cannons, we would do</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>all_cannons = Cannon.objects.all()
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</pre></div>
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</div>
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<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 different typeclasses. You won’t find any <code class="docutils literal notranslate"><span class="pre">Cannon</span></code> instances in
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the <code class="docutils literal notranslate"><span class="pre">all_weapon</span></code> result above, confusing as that may sound. To get instances of a Typeclass <em>and</em> the
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instances of all its children classes you need to use <code class="docutils literal notranslate"><span class="pre">_family</span></code>:</p>
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<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
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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
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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
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what you expect.</p>
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<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
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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
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query:</p>
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<aside class="sidebar">
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<p class="sidebar-title">_family</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>The all_family, filter_family etc is an Evennia-specific
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thing. It's not part of regular Django.
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</pre></div>
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</div>
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<p>The all_family, filter_family etc is an Evennia-specific
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thing. It’s not part of regular Django.</p>
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</aside>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>really_all_weapons = Weapon.objects.all_family()
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</pre></div>
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</div>
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<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.</p>
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<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>
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<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
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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
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<code class="docutils literal notranslate"><span class="pre">Sword</span></code>).</p>
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<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>
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(or <code class="docutils literal notranslate"><span class="pre">.filter_family</span></code>) instead:</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>roses = Flower.objects.filter(db_key="rose")
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</pre></div>
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</div>
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<p>This is a queryset representing all objects having a <code class="docutils literal notranslate"><span class="pre">db_key</span></code> equal to <code class="docutils literal notranslate"><span class="pre">"rose"</span></code>.
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<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">"rose"</span></code>.
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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>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>local_roses = roses.filter(db_location=myroom)
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</pre></div>
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@ -118,8 +121,10 @@ Since this is a queryset you can keep adding to it; this will act as an <code cl
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>local_non_red_roses = local_roses.exclude(db_key="red_rose")
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</pre></div>
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</div>
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<p>Only until we actually try to examine the result will the database be called. Here it’s called when we
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try to loop over the queryset:</p>
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<p>It’s important to note that we haven’t called the database yet! Not until we
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actually try to examine the result will the database be called. Here the
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database is called when we try to loop over it (because now we need to actually
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get results out of it to be able to loop):</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>for rose in local_non_red_roses:
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print(rose)
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</pre></div>
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@ -127,9 +132,20 @@ try to loop over the queryset:</p>
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<p>From now on, the queryset is <em>evaluated</em> and we can’t keep adding more queries to it - we’d need to
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create a new queryset if we wanted to find some other result. Other ways to evaluate the queryset is to
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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>
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<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
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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
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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>
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<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
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database fields. By convention Evennia uses <code class="docutils literal notranslate"><span class="pre">db_</span></code> in front of every database
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field. When you use the normal Evennia search helpers and objects you can skip
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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
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‘real’ names.</p>
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<aside class="sidebar">
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<p class="sidebar-title">database fields</p>
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<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
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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
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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
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only need to remember the <code class="docutils literal notranslate"><span class="pre">db_</span></code> when using them in database queries. The object
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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
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arbitrary Attributes attached to the Object.</p>
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</aside>
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<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>
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<ul class="simple">
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<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
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@ -156,7 +172,7 @@ roses = Flower.objects.filter(db_key__exact="rose"
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roses = Flower.objects.filter(db_key__iexact="rose")
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</pre></div>
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</div>
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<p>The Django field query language uses <code class="docutils literal notranslate"><span class="pre">__</span></code> in the same way as Python uses <code class="docutils literal notranslate"><span class="pre">.</span></code> to access resources. This
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<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
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is because <code class="docutils literal notranslate"><span class="pre">.</span></code> is not allowed in a function keyword.</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>roses = Flower.objects.filter(db_key__icontains="rose")
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</pre></div>
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@ -168,7 +184,8 @@ comparisons (same for <code class="docutils literal notranslate"><span class="pr
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>swords = Weapons.objects.filter(db_key__in=("rapier", "two-hander", "shortsword"))
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</pre></div>
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</div>
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<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 identify mages:</p>
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<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
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this is how we have identified mages:</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>char.tags.add("mage", category="profession")
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</pre></div>
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</div>
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@ -188,7 +205,7 @@ comparisons (same for <code class="docutils literal notranslate"><span class="pr
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<section id="get-that-werewolf">
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<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>
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<p>Let’s see if we can make a query for the werewolves in the moonlight we mentioned at the beginning
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of this section.</p>
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of this lesson.</p>
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<p>Firstly, we make ourselves and our current location match the criteria, so we can test:</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>> py here.tags.add("moonlit")
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> py me.db.lycantrophy = 3
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@ -198,11 +215,9 @@ of this section.</p>
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possible.</p>
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<aside class="sidebar">
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<p class="sidebar-title">Line breaks</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Note the way of writing this code. It would have been very hard to read if we just wrote it in
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one long line. But since we wrapped it in `(...)` we can spread it out over multiple lines
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without worrying about line breaks!
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</pre></div>
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</div>
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<p>Note the way of writing this code. It would have been very hard to read if we
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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
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it out over multiple lines without worrying about line breaks!</p>
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</aside>
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<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>
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@ -211,25 +226,26 @@ without worrying about line breaks!
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<span class="o">.</span><span class="n">filter</span><span class="p">(</span>
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<span class="n">db_location__db_tags__db_key__iexact</span><span class="o">=</span><span class="s2">"moonlit"</span><span class="p">,</span>
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<span class="n">db_attributes__db_key</span><span class="o">=</span><span class="s2">"lycantrophy"</span><span class="p">,</span>
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<span class="n">db_attributes__db_value__gt</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
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<span class="n">db_attributes__db_value__gt</span><span class="o">=</span><span class="mi">2</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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<ul class="simple">
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<li><p><strong>Line 3</strong> - 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>
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<li><p><strong>Line 4</strong> - We start to filter …</p></li>
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<li><p><strong>Line 5</strong></p>
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<ul>
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<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>
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<li><p>We start to filter …</p></li>
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<li><ul>
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<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>
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<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
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that we can treat like an object for this purpose; it references all Tags on the location)</p></li>
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<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>
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</ul>
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</li>
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<li><p><strong>Line 6</strong> - … 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">"lycantrophy"</span></code></p></li>
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<li><p><strong>Line 7</strong> - … 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>
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<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">"lycantrophy"</span></code></p></li>
|
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<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>
|
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</ul>
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||||
<p>Running this query makes our newly lycantrrophic Character appear in <code class="docutils literal notranslate"><span class="pre">will_transform</span></code>. Success!</p>
|
||||
<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>
|
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<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
|
||||
|
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@ -265,10 +281,12 @@ Character.objects.filter(q1 | ~q2)
|
|||
</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!). Let’s say there is
|
||||
a Tag “recently_bitten” that controls this.</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>
|
||||
|
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|
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@ -301,21 +319,22 @@ a Tag “recently_bitten” that controls this.</p>
|
|||
</div>
|
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<aside class="sidebar">
|
||||
<p class="sidebar-title">SQL</p>
|
||||
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>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 `LEFT OUTER JOIN`,
|
||||
which may lead to multiple merged rows combining the same object with different relations.
|
||||
</pre></div>
|
||||
</div>
|
||||
<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>
|
||||
<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>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>
|
||||
|
||||
|
|
@ -327,11 +346,13 @@ object? Maybe we want to find rooms only containing five or more objects?</p>
|
|||
<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>
|
||||
<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>
|
||||
|
||||
|
|
@ -350,17 +371,19 @@ that will count the number of results inside the database.</p>
|
|||
<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 5. This is a little
|
||||
harder to get one’s head around but much more efficient than lopping over all objects in Python.</p>
|
||||
<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>
|
||||
<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>
|
||||
|
||||
|
|
@ -420,10 +443,11 @@ in a format like the following:</p>
|
|||
</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>
|
||||
<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>
|
||||
|
||||
|
|
|
|||
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Reference in a new issue