django models - instance method to check if a model with matching attributes already exists in database

I am using django 2.0.8 and Python 3.5 in a project.

I want to write an instance method on class Foo that checks to ensure that a "similar record" (i.e. same attributes) does not exist in the database.

Foo

I am aware that I could use R.I (referential integrity) rules in the DB, to enforce this, but I want to push this responsibility to the model, not the DB layer.

Here is the relevant portion of my code:

class Foo(models.Model):
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')

# Model specific fields
user = models.ForeignKey(settings.AUTH_USER_MODEL, blank=False, null=False, default=1, on_delete = models.CASCADE)

def is_available(self):
return not models.Manager().get_queryset().filter(
content_type=self.content_type,
object_id=self.object_id,
user=self.user).exists()


My question is how to correctly implement the instance method is_available(). As it currently stands, when I invoke that method, I get the following error message (note Foo has been initialised correctly - omitted for sake of brevity):

is_available()

>>> foo.is_available()
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/path/to/project_base/myapp/models.py", line 57, in is_available
user=self.user).exists()
File "/path/to/env/lib/python3.5/site-packages/django/db/models/query.py", line 836, in filter
return self._filter_or_exclude(False, *args, **kwargs)
File "/path/to/env/lib/python3.5/site-packages/django/db/models/query.py", line 854, in _filter_or_exclude
clone.query.add_q(Q(*args, **kwargs))
File "/path/to/env/lib/python3.5/site-packages/django/db/models/sql/query.py", line 1253, in add_q
clause, _ = self._add_q(q_object, self.used_aliases)
File "/path/to/env/lib/python3.5/site-packages/django/db/models/sql/query.py", line 1277, in _add_q
split_subq=split_subq,
File "/path/to/env/lib/python3.5/site-packages/django/db/models/sql/query.py", line 1153, in build_filter
lookups, parts, reffed_expression = self.solve_lookup_type(arg)
File "/path/to/env/lib/python3.5/site-packages/django/db/models/sql/query.py", line 1015, in solve_lookup_type
_, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
File "/path/to/env/lib/python3.5/site-packages/django/db/models/sql/query.py", line 284, in get_meta
return self.model._meta
AttributeError: 'NoneType' object has no attribute '_meta'


filter()

temp is None

True

temp.exists()

return Foo.objects.filter(content_type=self.content_type, object_id=self.object_id, user=self.user).exists()

Foo.objects

1 Answer
1

You can't use a generic Manager() to do this, because it will not have the context of the model that you're trying to query. You can just use the model's defined manager (objects) instead - something like this:

Manager()

objects

def is_available(self):
return Foo.objects.filter(
content_type=self.content_type,
object_id=self.object_id,
user=self.user
).exists()


Note that doing this at the model level means you're not safe from race conditions, and it is still possible to end up saving "duplicate" objects to the database. Also,the result will change depending on whether the instance (self) has been saved or not - which might be an argument for doing this in a class method instead.

self

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