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PODjango Generic Relations and ORM Queries
 

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  1. PODjango Generic Relations and ORM Queries
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    <p>Say I have the following models:</p> <pre><code>class Image(models.Model): image = models.ImageField(max_length=200, upload_to=file_home) content_type = models.ForeignKey(ContentType) object_id = models.PositiveIntegerField() content_object = generic.GenericForeignKey() class Article(models.Model): text = models.TextField() images = generic.GenericRelation(Image) class BlogPost(models.Model): text = models.TextField() images = generic.GenericRelation(Image) </code></pre> <p>What's the most processor- and memory-efficient way to find all Articles that have at least one Image attached to them?</p> <p>I've done this:</p> <pre><code>Article.objects.filter(pk__in=Image.objects.filter(content_type=ContentType.objects.get_for_model(Article)).values_list('object_id', flat=True)) </code></pre> <p>Which works, but besides being ugly it takes forever.</p> <p>I suspect there's a better solution using raw SQL, but that's beyond me. For what it's worth, the SQL generated by the above is as following:</p> <pre><code> SELECT `issues_article`.`id`, `issues_article`.`text` FROM `issues_article` WHERE `issues_article`.`id` IN (SELECT U0.`object_id` FROM `uploads_image` U0 WHERE U0.`content_type_id` = 26 ) LIMIT 21 </code></pre> <p><strong>EDIT:</strong> czarchaic's suggestion has much nicer syntax but even worse (slower) performance. The SQL generated by his query looks like the following:</p> <pre><code>SELECT DISTINCT `issues_article`.`id`, `issues_article`.`text`, COUNT(`uploads_image`.`id`) AS `num_images` FROM `issues_article` LEFT OUTER JOIN `uploads_image` ON (`issues_article`.`id` = `uploads_image`.`object_id`) GROUP BY `issues_article`.`id` HAVING COUNT(`uploads_image`.`id`) &gt; 0 ORDER BY NULL LIMIT 21 </code></pre> <p><strong>EDIT:</strong> Hooray for Jarret Hardie! Here's the SQL generated by his should-have-been-obvious solution:</p> <pre><code>SELECT DISTINCT `issues_article`.`id`, `issues_article`.`text` FROM `issues_article` INNER JOIN `uploads_image` ON (`issues_article`.`id` = `uploads_image`.`object_id`) WHERE (`uploads_image`.`id` IS NOT NULL AND `uploads_image`.`content_type_id` = 26 ) LIMIT 21 </code></pre>
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    1. COIs this your actual model structure, or is there an entire class hierarchy that you aren't representing in your question example for simplicity's sake? I ask because this particular example doesn't require generics at all.
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    2. CONo, this is a stripped-down and simplified model structure.
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    3. COThough your accepted answer works quite well, I'm curious to know what the solution would be if you were requiring something other than 'at least one image'.
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