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]> 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 > >-- >You received this message because you are subscribed to the Google Groups >"Django users" group. >To unsubscribe from this group and stop receiving emails from it, send an >email to [email protected]. >To view this discussion on the web visit >https://groups.google.com/d/msgid/django-users/CAKL%3DYPH5OCd9wYNG%2BZ4R0%3DuYOEW0hfhRngqfudeaGqCjB6Yfrg%40mail.gmail.com. -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/mosb6bo4w6bv3axaaicmqsht.1563749793797%40email.android.com.
