Django model unique together both ways

Question

Many questions already on this topic, but not what i'm searching for.

I have this Model:

class Options(TimeStampedModel) option_1 = models.CharField(max_length=64) option_2 = models.CharField(max_length=64) class Meta: unique_together = ('option_1', 'option_2')

Now I have a unique constraint on the fields. Is there a way to also define this the other way around so that it doesn't matter what was option_1 and what was option_2

As example:

Options.create('spam', 'eggs') # Allowed Options.create('spam', 'eggs') # Not allowed Options.create('eggs', 'spam') # Is allowed but should not be

Thanks in advance!

This seems like a strange thing to want to do, what is your specific use case? — Azsgy
– Azsgy, Commented Apr 13, 2018 at 12:30
They are options.. and options shouldn't go together more than once, left or rightways — Nebulosar
– Nebulosar, Commented Apr 13, 2018 at 12:37
I feel a better solution might be a many to many relationship and making an Option model. — Azsgy
– Azsgy, Commented Apr 13, 2018 at 12:45
You could do that with a CHECK constraint on the database, but then you'll want more options and it will be messy... @etene's solution is more appropriate here — Kos
– Kos, Commented Apr 13, 2018 at 13:02
@Kos How would you do this using a CheckConstraint? I'd like to push more validation into the database itself, but using a ManyToMany relationship as in @etene's solution below is not an option. The problem I'm finding with CheckConstraint's is they seem to be limited to the data in the row being inserted. — johnthagen
– johnthagen, Commented Feb 23, 2023 at 19:54

Dean Elliott · Accepted Answer · 2019-10-31 10:23:19Z

I think a ManyToMany relation with a custom through table and an unique_together constraint on that table should do what you want.

Example code:

from django.db.models import Model, ForeignKey, ManyToManyField, CharField class Option(Model): name = CharField() class Thing(TimeStampedModel): options = ManyToManyField("Option", through="ThingOption") class ThingOption(Model): thing = ForeignKey(Thing) option = ForeignKey(Option) value = CharField() class Meta: unique_together = ('thing', 'option')

For Django 2.2+ it is recommended to use UniqueConstraint. In the docs there is a note stating unique_together may be deprecated in the future. See this post for its usage.

I'm struggling to understand this. The options are a field on the Thing model. So I can select the 'spam' and 'eggs' Option models in that? How does the ThingOption table prevent the selection the other way around? And what is value?
This is a bit old for me, but you should probably read the docs I've linked, esp. on many-to-many through tables. The ThingOption table doesn't prevent selection in any way, it just prevents setting the same Option more than once for the same Thing, through the unique_together constraint.

ramganesh · Accepted Answer · 2018-04-13 12:39:14Z

You can override create method, do something like

from django.db import models class MyModelManager(models.Manager): def create(self, *obj_data): # Do some extra stuff here on the submitted data before saving... # Ex- If obj_data[0]=="eggs" and obj_data[1]=="spam" is True don't allow it for your blah reason # Call the super method which does the actual creation return super().create(*obj_data) # Python 3 syntax!! class MyModel(models.model): option_1 = models.CharField(max_length=64) option_2 = models.CharField(max_length=64) objects = MyModelManager()

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