M2M relationship not behaving as I would expect

[Don Baldwin]

Hi,

I have a many-to-many relationship between users and things, where a user can select multiple things,
and a thing can be selected by multiple users.  The relationship between users and things also
contains data, so I've specifically setup a through table.

When I add a user to a thing, I guess I expect a one-to-one relationship between the thing and
the intervening bridge, but it seems like I'm getting a one-to-many.

Here is my code:

models.py:

    class TestUser(models.Model):
        user_data = models.TextField(default="")
    
        def __str__(self):
            return "Other: " + self.user_data
    
    class TestThing(models.Model):
        thing_data = models.TextField(default="")
        user = models.ManyToManyField(TestUser, through='TestBridge')
    
        def __str__(self):
            return "Thing: " + self.thing_data
    
    class TestBridge(models.Model):
        user = models.ForeignKey(TestUser, on_delete=models.CASCADE)
        thing = models.ForeignKey(TestThing, on_delete=models.CASCADE)
        bridge_data = models.TextField(default="")
    
        def __str__(self):
            return "Bridge: " + self.bridge_data

tests.py:

        u_1 = TestUser(user_data = 'user')
        u_1.save()
        t_1 = TestThing(thing_data='thing 1')
        t_1.save()
        t_2 = TestThing(thing_data='thing 2')
        t_2.save()

        t_1.user.add(u_1, through_defaults={'bridge_data': 'bridge 1'})
        t_2.user.add(u_1, through_defaults={'bridge_data': 'bridge 2'})

        data = list(TestThing.objects.all().values('user__user_data',
                                                   'thing_data',
                                                   'user__testbridge__bridge_data'))
        for item in data:
            print(item)

Output:

    {'user__user_data': 'user', 'thing_data': 'thing 1', 'user__testbridge__bridge_data': 'bridge 1'}
    {'user__user_data': 'user', 'thing_data': 'thing 1', 'user__testbridge__bridge_data': 'bridge 2'}
    {'user__user_data': 'user', 'thing_data': 'thing 2', 'user__testbridge__bridge_data': 'bridge 1'}
    {'user__user_data': 'user', 'thing_data': 'thing 2', 'user__testbridge__bridge_data': 'bridge 2'}
    
What I expect:

    {'user__user_data': 'user', 'thing_data': 'thing 1', 'user__testbridge__bridge_data': 'bridge 1'}
    {'user__user_data': 'user', 'thing_data': 'thing 2', 'user__testbridge__bridge_data': 'bridge 2'}

How do I get rid of the relationships between thing 1 and bridge 2, and between thing 2 and bridge 1?

Thanks for your responses.

-Don

[Mike Dewhirst]

You can constrain uniqueness in the through table and/or the other tables to represent whatever real-world constraints you need. Otherwise you can have as many duplicated relationships as you want. Each through record has its own id so the database is happy no matter how many go in.

Connected by Motorola

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[Don Baldwin]

FYI, I was able to get this working with a small change to the query, from:

data = list(TestThing.objects.all().values('user__user_data',

                                           'thing_data',

                                           'user__testbridge__bridge_data'))

to:

data = list(TestThing.objects.all().values('user__user_data',

                                           'thing_data',

                                           'testbridge__bridge_data'))

(removing the highlighted code).

Somehow this extra level of indirection caused the query to lose the constraints of the one-to-many relationships between user and bridge and between thing and bridge, although I don't fully understand it.

Thanks for your response Mike.

-Don

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