django check-contraint boolean expression [postgresql] - django

Question

So I have a model like this
class Role(BaseModel):
class Meta:
verbose_name = 'role'
verbose_name_plural = 'roles'
ordering = ['position', 'cluster']
required_db_features = {
'supports_deferrable_unique_constraints',
}
constraints = [
models.UniqueConstraint(
fields=['position', 'cluster'],
name='deferrable_unique_role_position',
deferrable=models.Deferrable.DEFERRED
),
models.CheckConstraint(
name='default_role_check',
check=models.Q(
is_default=True,
position=1,
color='#969696',
name='#everyone'
)
)
]
permission_flags = [
'READ_DATA', 'WRITE_DATA', 'MANAGE_RECORDS', 'MANAGE_ROLES',
'MANAGE_CLUSTER', 'MANAGE_DATASHEETS', 'MANAGE_FIELDS', 'MANAGE_CONSTRAINTS',
'KICK_MEMBERS', 'MANAGE_MEMBERS',
]
default_perm_flags = ['READ_DATA', 'WRITE_DATA', 'MANAGE_RECORDS']
def __str__(self):
return self.name
objects = managers.RolesManager()
positions = managers.PositionalManager()
permissions = BitField(flags=permission_flags, default=default_perm_flags, db_index=True)
position = models.PositiveSmallIntegerField(null=True, blank=True, db_index=True, editable=True)
name = models.CharField(max_length=100, validators=[MinLengthValidator(2)], db_index=True, default='new role')
color = ColorField(db_index=True, default='#969696')
is_default = models.BooleanField(default=False, db_index=True)
cluster = models.ForeignKey('api_backend.Cluster', on_delete=models.CASCADE, editable=False)
Basically I want to enforce a check constraint on this model such that
when the model has the is_default field set to True,
the name must always be #everyone
the color must always be #969696
the position must always be 1.
I tried to implement the same, however my current implementation doesn't work.
While using serializers, I can edit the default role's name, and no exceptions are raised.
I am using postgresql at the backend.
Can someone please help me?
thanks in advance!

Answer 1

Adding an is_default=False case get you closer to your conditions.
I tested with Django==3.1.4 and the OR method resulted in this constraint:
"default_role_check" CHECK (color::text = '#969696'::text AND is_default AND name::text = '#everyone'::text AND "position" = 1 OR NOT is_default)
Constraint example:
class Role(BaseModel):
class Meta:
verbose_name = 'role'
verbose_name_plural = 'roles'
ordering = ['position', 'cluster']
required_db_features = {
'supports_deferrable_unique_constraints',
}
constraints = [
models.UniqueConstraint(
fields=['position', 'cluster'],
name='deferrable_unique_role_position',
deferrable=models.Deferrable.DEFERRED
),
models.CheckConstraint(
name='default_role_check',
check=(models.Q(
is_default=True,
position=1,
color='#969696',
name='#everyone'
) |
models.Q(
is_default=False,
)
)
)
]
permission_flags = [
'READ_DATA', 'WRITE_DATA', 'MANAGE_RECORDS', 'MANAGE_ROLES',
'MANAGE_CLUSTER', 'MANAGE_DATASHEETS', 'MANAGE_FIELDS', 'MANAGE_CONSTRAINTS',
'KICK_MEMBERS', 'MANAGE_MEMBERS',
]
default_perm_flags = ['READ_DATA', 'WRITE_DATA', 'MANAGE_RECORDS']
def __str__(self):
return self.name
objects = managers.RolesManager()
positions = managers.PositionalManager()
permissions = BitField(flags=permission_flags, default=default_perm_flags, db_index=True)
position = models.PositiveSmallIntegerField(null=True, blank=True, db_index=True, editable=True)
name = models.CharField(max_length=100, validators=[MinLengthValidator(2)], db_index=True, default='new role')
color = ColorField(db_index=True, default='#969696')
is_default = models.BooleanField(default=False, db_index=True)
cluster = models.ForeignKey('api_backend.Cluster', on_delete=models.CASCADE, editable=False)