在Django模型上，如何在ManyToMany字段中检查值集中的唯一性？

Question

For example if I have a set of models like this, how can I make sure that only one Group instance with the same exact set of Permission s could exist in the model?

class Permission(models.Model):
    name = models.CharField(max_length=100, unique=True)

class Group(models.Model):
    name = models.CharField(max_length=100, unique=True)
    permissions = models.ManyToManyField(Permission)

class User(models.Model):
    name = models.CharField(max_length=100, unique=True)
    group = models.ForeignKey(Group)

What is the best way to enforce this constraint in django? I don't care about a DB-level constraint. Does django provide an existing flag on the ManyToMany model field or I need to add custom data validation? And if yes, how?

Also I don't use ModelForm s, so form validation is not what I want.

My question is about uniqueness of set of ManyToMany field relations across the whole model, not in a single instance.

Answer 1

You can override the Group model's save method in order to check uniqueness before saving Group objects.

For example:

class Group(models.Model):
    ...
    def save(self, *args, **kwargs):
        if insert_your_check_here():
            super(Group, self).save(*args, **kwargs) # Call the "real" save() method.
        else:
            return

For more information about overriding predefined model methods take a look at the docs .