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PODjango query output from backwards ForeignKey relation
 

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  1. PODjango query output from backwards ForeignKey relation
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    <p>I have a django Model, <code>Foo</code>, which has various known and consistently-named attributes (<code>alpha</code>, <code>beta</code>, ...) as well as a variable number of instances of <code>Bar</code>; <code>Bar</code> is itself a model with attributes (<code>name</code>, <code>value</code>, ...). I can't have each <code>bar</code>'s name and value etc. as a column in the <code>Foo</code> model because there are too many of them, they have many different names, and some apply to only a few foos. So I have a ForeignKey relationship within the <code>Bar</code> model, so each <code>bar</code> knows which <code>foo</code> it belongs to, and I can use Django's <code>select_related()</code>, e.g.:</p> <pre><code>foos = Foo.objects.filter(alpha__lte=10).select_related() for foo in foos: bars = foos.bar_set.filter(name__in=('prop1', 'prop2', 'prop5')) for bar in bars: print '%s = %d' % (bar.name, bar.value) </code></pre> <p>This seems to work... but is it really the best way to organise my data with MySQL?</p> <p>If I'm on the right track, I have another question: how can I speed this up? I should add that I have 2,700,000 <code>Foos</code> and 16,900,000 <code>Bars</code> in total (with indexes in place, etc.). Particularly slow is my output - I'd like to be able to write text files with columns of the properties: <code>foo.alpha</code>, <code>foo.beta</code>, <code>foo.bar1.value</code>, <code>foo.bar2.value</code>, etc... where <code>bar1</code>, <code>bar2</code>, etc. are chosen by their <code>name</code> attributes. I've been experimenting with a list of output fields:</p> <pre><code>bar_output_fields = ['prop1', 'prop2', 'prop5'] all_output_fields = [('%4d', 'alpha'), (%12.6f', 'prop1'), (%10.3e', 'beta'), (%10.3e', 'prop2') ('%2d', 'prop5')] fo = open('output.txt', 'w') for foo in foos: bars = foos.bar_set.filter(name__in=bar_output_fields) for bar_output_field in bar_output_fields: bar = bars.filter(name=bar_output_field).get() setattr(foo, bar_output_field, bar.value) for (fmt, name) in all_output_fields: print &gt;&gt;fo, fmt % getattr(foo, name), print &gt;&gt;fo fo.close() </code></pre> <p>But this is extremely slow when the query returns >1000s of <code>foo</code>s. Anyone got any suggestions for improving performance here? I've a feeling I'm missing something big...</p>
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    1. COCheck out what SQL is being generated here: https://docs.djangoproject.com/en/dev/faq/models/#how-can-i-see-the-raw-sql-queries-django-is-running I think you may be executing a lot of sql queries, and you may actually get much better performance by doing some things in Python
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