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PODB / performance: layout of django model that rarely refers to its parent more than once
 

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  1. PODB / performance: layout of django model that rarely refers to its parent more than once
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    <p>I have an app that's about presenting fictional simplified cities.</p> <p>Please consider the following Django models:</p> <pre><code>class City(models.Model): name = models.CharField(...) ... TYPEGROUP_CHOICES = ( (1, 'basic'), (2, 'extra'), ) class BldgType(models.Model): name = models.CharField(...) group = models.IntegerField(choices=TYPEGROUP_CHOICES) class Building(models.Model): created_at = models.DateTimeField(...) city = models.ForeignKey(City) type = models.ForeignKey(BldgType) other_criterion = models.ForeignKey(...) class Meta: get_latest_by = 'created_at' </code></pre> <p>Explanations for choosing this setup:</p> <p>(1) Each city has certain buildings of a "basic" type which occur exactly once per city (examples: city hall, fire station, police station, hospital, school) and possibly dozens of buildings of "extra" type, such as dance clubs.</p> <p>(2) In certain views, all buildings (regardless of city, etc.) are to be filtered according to different criteria, e.g., <code>other_criterion</code>.</p> <p><b>Problem/concern:</b></p> <p>In a <code>city_detail</code> view, I would have to loop over any buildings of "extra" type, which is OK and normal.</p> <p>But I am not sure how to efficiently retrieve the city's "hospital" building, which is of "basic" type, so I must do this for every city anyway because exactly one such hospital exists in each city (this is ensured at city creation time).</p> <p>There will be at most a dozen of "basic" building types, of which about half will be presented all the time. </p> <p>I'm inclined towards writing convenience methods on the City model, and I face three options:</p> <p>(A1) Via <code>try</code> and index: <code>.filter(...)[0]</code></p> <p>(A2) Via <code>try</code> and <code>.get(...)</code></p> <p>(A3) Via <code>try</code> and <code>.filter(...).latest()</code></p> <p>But none of those seem elegant. Or is one of these three options good to combine with some sort of caching, like in Django's <code>get_profile()</code> method on the <code>User</code> model? Unfortunately, I have no experience with caching yet.</p> <p>Is it nuts to use the following option?</p> <p>(B) specific FKs in the City model, one for each of the most important basic types</p> <p><b>Question:</b></p> <p>Which option makes sense the most?<br> Or is the schema generally faulty for this kind of scenario?</p> <p>Especially regarding DB performance, what do you suggest? Do I need a completely different approach?</p> <p>Please advise! :)</p> <p>Thanks in advance!</p>
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