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    <p>If you don't want to actively pull the user set as dm03514 showed, such as if you want to add users to the thread by default but maintain the ability to remove them from the thread many-to-many later, you can indeed do this by overriding the <code>save</code> method or by using a <code>post_save</code> signal.</p> <p><code>save</code> is good enough for almost all cases - the advantage of <code>post_save</code> is that it can more reliably distinguish between saving a new message and saving edits to an existing message. But if you're not creating messages with preselected PKs or loading them from fixtures <code>save</code> can work fine:</p> <pre><code>class Message(models.Model): def save(self, *args, **kwargs): probably_new = (self.pk is None) super(Message, self).save(*args, **kwargs) if probably_new: self.thread.user.add(self.sender) </code></pre> <p>A signal would look like this:</p> <pre><code>from django.db.models.signals import post_save def update_thread_users(sender, **kwargs): created = kwargs['created'] raw = kwargs['raw'] if created and not raw: instance = kwargs['instance'] instance.thread.user.add(instance.sender) post_save.connect(update_thread_users, sender=Message) </code></pre> <p>And then review the docs on preventing duplicate signals in case of multiple imports: <a href="https://docs.djangoproject.com/en/dev/topics/signals/#preventing-duplicate-signals" rel="nofollow">https://docs.djangoproject.com/en/dev/topics/signals/#preventing-duplicate-signals</a></p>
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    1. POSave the user from one model to the another model
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    1. USPeter DeGlopper
    1. USPeter DeGlopper
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    1. COJust what I wanted, the `save` method! Thank you!
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      1. USAamu
    2. COI am sorry could you please elaborate what this means, `creating messages with preselected PKs or loading them from fixtures`. I want to know what drawbacks the save method will have.
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      1. USAamu
    3. COIt is possible in Django, although rare, to set the primary key of a new object prior to saving it. The above `save` method would then not detect that it's creating a new object rather than editing an existing one. See here: https://docs.djangoproject.com/en/dev/ref/models/instances/#explicitly-specifying-auto-primary-key-values The code that triggers signals makes an extra database call to determine this, so `created` is more reliable. Fixtures means using the `loaddata` manage.py command to import data from a file. Sometimes you don't want to run `save` side effects in the middle of that.
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      1. USPeter DeGlopper
 

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