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PODjango query — how to get list of dictionaries with M2M relation?
 

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  1. PODjango query — how to get list of dictionaries with M2M relation?
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    <p>Let's say, I have this simple application with two models — <code>Tag</code> and <code>SomeModel</code></p> <pre><code>class Tag(models.Model): text = ... class SomeModel(models.Model): tags = models.ManyToManyField(Tag, related_name='tags') </code></pre> <p>And I want to get something like this from database:</p> <pre><code>[{'id': 1, 'tags': [1, 4, 8, 10]}, {'id': 6, 'tags': []}, {'id': 8, 'tags': [1, 2]}] </code></pre> <p>It is list of several SomeModel's dictionaries with SomeModel's id and ids of tags.</p> <p>What should the Django query looks like? I tried this:</p> <pre><code>&gt;&gt;&gt; SomeModel.objects.values('id', 'tags').filter(pk__in=[1,6,8]) [{'id': 1, 'tags': 1}, {'id': 1, 'tags': 4}, {'id': 1, 'tags': 8}, ...] </code></pre> <p>This is not what I want, so I tried something like this:</p> <pre><code>&gt;&gt;&gt; SomeModel.objects.values_list('id', 'tags').filter(pk__in=[1,6,8]) [(1, 1), (1, 4), (1, 8), ...] </code></pre> <p>And my last try was:</p> <pre><code>&gt;&gt;&gt; SomeModel.objects.values_list('id', 'tags', flat=True).filter(pk__in=[1,6,8]) ... TypeError: 'flat' is not valid when values_list is called with more than one field. </code></pre> <p>— </p> <p>Maybe Django cannot do this, so the most similar result to what I want is:</p> <pre><code>[{'id': 1, 'tags': 1}, {'id': 1, 'tags': 4}, {'id': 1, 'tags': 8}, ...] </code></pre> <p>Is there any Python build-in method which transform it to this?</p> <pre><code>[{'id': 1, 'tags': [1, 4, 8, 10]}, {'id': 6, 'tags': []}, {'id': 8, 'tags': [1, 2]}] </code></pre> <p><strong>— EDIT:</strong></p> <p>If I write method in <code>SomeModel</code>:</p> <pre><code>class SomeModel(models.Model): tags = models.ManyToManyField(Tag, related_name='tags') def get_tag_ids(self): aid = [] for a in self.answers.all(): aid.append(a.id) return aid </code></pre> <p>And then call:</p> <pre><code>&gt;&gt;&gt; sm = SomeModel.objects.only('id', 'tags').filter(pk__in=[1,6,8]) # Hit database &gt;&gt;&gt; for s in sm: ... s.get_tag_ids() ... &gt;&gt;&gt; # Hit database 3 times. </code></pre> <p>This is not working, because it access to database 4 times. I need just one access.</p>
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    1. CONot sure if this helps, but have you looked at [django-tagging](http://code.google.com/p/django-tagging/) or [django-taggit](https://github.com/alex/django-taggit)? If you're not necessarily coding tags, you may be able to glean some code from them for how they handle it.
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      1. USRob
    2. COI know django-tagging, not django-taggit. Thank you, but this is just my hobby project and I want to learn Django as much as possible, so I don't want to use any third-party code.
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      1. USvasco
    3. COYou can always just write your own classmethod or Manager to return the data in the format you're needing.
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      1. USBrandon
 

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