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POUsing a Proxy Model when accessing foreign key
 

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    <p>I have two related Django models. One of the models does expensive calculations in its <code>__init__</code> which I can't move elsewhere without unacceptable cost/risk. </p> <p>It turns out these expensive calculations aren't needed in all contexts, so I introduced a proxy model that circumvents them. They are needed more often than not, though, so it's not practical to make the expensive one into the proxy.</p> <p>So, my code basically looks like this:</p> <pre><code>class Person(models.Model): def __init__(self, *args, **kw): models.Model.__init__(self, *args, **kw) do_some_really_expensive_things() class LightweightPerson(Person): class Meta: proxy = True def __init__(self, *args, **kw): models.Model.__init__(self, *args, **kw) class PersonFact(models.Model): fact = models.TextField() person = models.ForeignKey(Person) </code></pre> <p>This works well -- most of my code queries on <code>Person</code>. And in the few places where the code doesn't need the really expensive things, it queries on <code>LightweightPerson</code> instead, and performs better.</p> <p>However, some of my code starts from <code>PersonFact</code> instances and accesses the related <code>person</code> for each <code>PersonFact</code>. This code doesn't need the really expensive person calculations, and the performance hit from those expensive calculations is unacceptable. So I'd like to be able to instantiate a <code>LightweightPerson</code> instead of a <code>Person</code> in this context.</p> <p>The approach I came up with was to add a second <code>ForeignKey</code> that references the proxy class, and uses the same database column:</p> <pre><code>class PersonFact(models.Model): fact = models.TextField() person = models.ForeignKey(Person, db_column="person_id") lightweight_person = models.ForeignKey( LightweightPerson, db_column="person_id", related_name="lightweight_personfact_set") </code></pre> <p>So now when I need the performance boost, my code can do things like this:</p> <pre><code>facts = PersonFact.objects.select_related( "lightweight_person").all() for fact in facts: do_something_with(fact.lightweight_person) </code></pre> <p>And this works great! Until I try to save a new <code>PersonFact</code>:</p> <pre><code>&gt;&gt;&gt; fact = PersonFact(fact="I like cheese", person=some_guy_i_know) &gt;&gt;&gt; fact.save() Traceback (most recent call last): ... DatabaseError: column "person_id" specified more than once </code></pre> <p>:-(</p> <p>Is there any way to do this without a big scary refactoring of the code that's currently in <code>Person.__init__</code>? Ideally I'd either be able to just signal "when accessing <code>person_fact.person</code> right now, please instantiate a <code>LightweightPerson</code> instead of a <code>Person</code>" in my calling code. Or, alternatively, I'd like to be able to declare a "proxy related field" on <code>PersonFact</code> that shadows the same database column, but Django's internals know to only interact once with the database column.</p>
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    1. COI had a similar discussion in #django in IRC and basically they told me what I'm doing is considered a bug. It's kind of frustrating when sometimes this is the only viable solution.
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      1. USMike Shultz
 

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