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copied!<p>There are a couple approaches you can take to handling the <code>QuerySet</code>s of "mixed models" that you obtain when you perform this kind of query. A lot of it depends on your ultimate goal.</p> <p>The "simple and dumb" way, which I use a lot, is to manage the results with utility functions. If you plan to process the result of <code>Build.objects.filter(project="myproject")</code> in a template, for example, you could use custom template tags or filters to take special action. Assume in the below code <code>build_objects</code> contains the result of your filter():</p> <pre><code>{% for build in build_objects %} {% if build|is_custom_build %} <p>This is a custom build of custom type {{ build.custom_build.custom_type }}!</p> {% endif %} <p>This is a custom build OR a standard build</p> {% endfor %} </code></pre> <p>The obvious problem here is if you have numerous subclasses, writing template filters may be impractical or grow tedious. However, in my experience I usually have a half-dozen subclasses at most so this is not always a problem.</p> <p>You could also write a parameterized filter like so:</p> <pre><code>{% if build|is_of_buildtype:"custom_build" %} </code></pre> <p>With the filter code as follows:</p> <pre><code>def is_of_buildtype_filter(value, arg): if hasattr(value, arg): return True else: return False </code></pre> <p>This filter simply checks for the presence of the argument as an attribute on the <code>build</code> object (passed in as <code>value</code>). The argument string should be the name of the automatically generated <code>OneToOneField</code> that you want to detect, in this case <code>custom_build</code>.</p> <p>For view code, similar sorts of helper functions would work the same way but are even easier because you don't need to write custom filters or tags.</p> <p>This approach works in many cases, but there are more complicated scenarios where it might not be practical. Unfortunately, Django cannot natively provide you with a <code>QuerySet</code> that contains subclass instances when you perform operations on the base class (ie. a <code>QuerySet</code> that truly contains "mixed models"). You may require this in situations where processing the results with helper functions is not possible.</p> <p>I personally avoid these situations entirely, usually by rethinking my model design. But if that's not possible, there are many interesting attempts at solutions, like this <a href="http://lazypython.blogspot.com/2009/02/second-look-at-inheritance-and.html" rel="nofollow noreferrer">Inheritance MixIn</a>. There are also several Django snippets on the subject. Be aware, though, that almost any solution like this will be performance-limited.</p>

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