Django UniqueConstraint doesn't works with ManyToManyField, got exception FieldDoesNotExist

Question

The migration stopped working when switching from ForeignKey between Shop and user to a ManyToManyField. I wanted shops to be able to be owned by different users at the same time:

class Shop(models.Model):
    name = models.CharField('name', max_length=120)
    #user = models.ForeignKey(User, on_delete=models.CASCADE)   ##before
    shopuser= models.ManyToManyField(User, related_name="shopusers", blank=True)    ##after

    class Meta:
        constraints = [models.UniqueConstraint(fields=['shopuser', 'name'], name='user cant have the same shop twice!')] 
    
    ## after:
    @property
    def get_shopuser(self):
        return ", ".join([u.username for u in self.shopuser.all()])

class Warehouse(models.Model):
    address = models.CharField('address', max_length=120)
    user = models.ManyToManyField(User, related_name="User", blank=True)

django.core.exceptions.FieldDoesNotExist: NewShop has no field named 'shopusers'

I thought by choosing a related name I can use multiple relations to the User model? I already tried completely deleting my database (and migrations folder) and migrate from start, but tht did not help :(

Example where I would call the code:

admin.py:

@admin.register(Shop)
class ShopAdmin(admin.ModelAdmin):
    list_display = ("name", "related_shopuser")
    list_filter = ("name", "shopuser")
    fieldsets = [("Requrired Information", {"description": "These fields are required", 
                                            "fields": (("name", "shopuser"))}),]

    def related_shopuser(self, obj):
        return obj.get_shopuser

Where does Djanog migrations get the NewShop from FieldDoesNotExist("%s has no field named '%s'" % (self.object_name, field_name))? Is it automatically generated from the ModelName Shop?

Answer 1

Issue

The UniqueConstraint on ManyToManyField will not work as expected. If you want to enforce the constraint, you should define a intermediate model and connect them using the through--(Django doc) parameter.

For the sake of simplicity, I assume you have a model as below,

class Shop(models.Model):
    name = models.CharField('name', max_length=120)
    user = models.ForeignKey(User, on_delete=models.CASCADE)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=[
                    'user',
                    'name'
                ],
                name='user cant have the same shop twice!'
            )
        ]

and now you want make the user field to a ManyToManyField from ForeignKey

Note: I have changed the field name to users from user, which is more approriate.

Method-1 : Remove unique constraint

class Shop(models.Model):
    name = models.CharField('name', max_length=120)
    users = models.ManyToManyField(User)

Now, run makemigrations and migrate commands

Method-2: use an intermediate model

class ShopUserIntermediateModel(models.Model):
    shop = models.ForeignKey('Shop', models.CASCADE)
    user = models.ForeignKey(User, models.CASCADE)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=[
                    'shop',
                    'user'
                ],
                name='user cant have the same shop twice!'
            )
        ]


class Shop(models.Model):
    name = models.CharField('name', max_length=120)
    users = models.ManyToManyField(User, through=ShopUserIntermediateModel)