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
On Sunday, July 21, 2019 at 3:58:03 PM UTC-7, Mike Dewhirst wrote:
>
> 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*
>
>
> Don Baldwin <[email protected] <javascript:>> wrote:
>
> 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
>
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