I want to have two foreign keys to the same model:
class Test(models.model):
example1 = models.ForeignKey(Example)
example2 = models.ForeignKey(Example)
I get errors like:
Accessor for field 'example1' clashes with related
field 'Example.test_set'. Add a related_name argument
to the definition for 'example1'.
Try using related_name:
class Test(models.model):
example1 = models.ForeignKey('Example', related_name='example1')
example2 = models.ForeignKey('Example', related_name='example2')
Django uses some python magic to define relationships between models, some of which involves using the name of the models in the relationships (that's where the 'test' in 'test__set' is coming from.) What's happening, I would guess, is that it's trying to put "test__set" in the Example model twice, once for each foreign key you've got defined.
The error message suggests something to try: define a related_name argument (overriding one of those 'test_set's) that it can use instead of auto-generating two clashing names.
More info here: page has been removed
Current page relating to model relationships:
https://docs.djangoproject.com/en/2.0/ref/models/fields/#module-django.db.models.fields.related
Just do what the error message tells you to do, and if you're unsure what that means, consult the documentation for related_name.
In django 2.0 Try this:
user = models.ForeignKey(User, on_delete=models.PROTECT, null=True, related_name='user')
paper = models.ForeignKey(paperRecord, on_delete=models.PROTECT, null=True, related_name='paper')
Related
I'd like to create a many-to-many relationship from and to a user class object.
I have something like this:
class MyUser(models.Model):
...
blocked_users = models.ManyToManyField(MyUser, blank=True, null=True)
The question is if I can use the class reference inside itself. Or do I have to use "self" insead of "MyUser" in the ManyToManyField? Or is there another (and better) way to do it?
Technically, I'm pretty sure "MyUser" or "self" will work, as long as it's a string in either case. You just can't pass MyUser, the actual class.
However, the docs always use "self". Using "self" is not only more explicit about what's actually happening, but it's impervious to class name changes. For example, if you later changed MyUser to SomethingElse, you would then need to update any reference to "MyUser" as well. The problem is that since it's a string, your IDE will not alert you to the error, so there's a greater chance of your missing it. Using "self" will work no matter what the class' name is now or in the future.
class MyUser(models.Model):
...
blocked_users = models.ManyToManyField("self", blank=True)
Don't forget use symmetrical=False, if you use .clear() or .add() method for related objects and don't wanna object on other side of relation update own data in relation field.
some_field = models.ManyToManyField('self', symmetrical=False)
I think it should be class name instead of self. because with using self like this
parent = models.ManyToManyField('self', null=True, blank=True)
when i add parent:
user1.parent.add(user2)
i have 2 record in database like this:
and with using class name liken this:
parent = models.ManyToManyField('User', null=True, blank=True)
i have one record in database like this:
note that i use uuid for pk and i use django 3.1
EDIT:
as #shinra-tensei explained as comment in this answer we have to set symmetrical to False if we use self. documented in Django Documents: ManyToManyField.symmetrical
If you use self or MyUser you will get a NameError in both cases. You should write "self" as string. See the example below:
class MyUser(models.Model):
...
blocked_users = models.ManyToManyField("self", blank=True, null=True)
And do not forget to set the symmetrical attribute to False if the relationship is not symmetrical.
For further details check: https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ManyToManyField
don't use 'self' in ManyToManyField, it will cause you object link each other, when use django form to submit it
class Tag(models.Model):
...
subTag = models.ManyToManyField("self", blank=True)
...
aTagForm.save()
and result:
a.subTag == b
b.subTag == a
I have two models like below in django
class User(AbstractBaseUser, PermissionsMixin):
first_name = models.CharField(....)
last_name = models.CharField(_(....)
email = models.EmailField(...)
class VcsToken(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
as you can see VcsToken is related to User, and a user can have many VcsToken,
So how do I get all the VcsTokens of a User.
Similarly I have many one to many relationships from user to other models, so How do I know their reference name? (I know its a set but how do I know the set name? ) Is there any way to list the query set names for a model.
What you are looking for is the feature for walking backward on the foreign key relationships. This is covered here in the docs: https://docs.djangoproject.com/en/3.0/topics/db/queries/#following-relationships-backward
In your example you should be able to access VcsToken from User like this:
user = User.objects.get(pk=1) # let's get an example user
user.vcstoken_set.all() # returns all related VcsToken objects.
Optionally, when the ForeignKey is defined, you can specify a related_name argument that would be used by django for this purpose. For example:
class User(AbstractBaseUser, PermissionsMixin):
...
email = models.EmailField(...)
class VcsToken(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='tokens')
If such an argument is specified, then that is the reverse lookup name used by django and you would need to do user.tokens.all() to access them. When no such argument is specified a default name is used by django that ends with "_set".
Hope this helps, let me know if anything needs to be clarified.
For a given User object myuser, you can access this with:
myuser.vcstoken_set.all()
so How do I know their reference name?
This is the value for the related_name=… parameter [Django-doc] if there is no such parameter in the ForeignKey construction, it will default to modelname_set, with the modelname in lowercase.
Is there any way to list the query set names for a model.
You can access all ManyToOneFields with:
django.db.models.fields.reverse_related.ManyToOneRel
[f.get_accessor_name() for f in User._meta.get_fields() if isinstance(f, ManyToOneRel)]
This will construct a list of the names of ForeignKeys in reverse.
I have this model:
class Person(models.Model):
something ...
employers = models.ManyToManyField('self', blank=True, related_name='employees')
When I do person.employees.all() I get this error: 'Person' object has no attribute 'employees'. Is the related name only created when there is an actual link in place. If yes, how can I check this?
EDIT: I'm aware of the hasattr() function. I'm still wondering why the attribute doesn't return an empty list when there's no related objects.
To use related_name with recursive many-to-many you need set symmetrical=False. Without it Django will not add employees attribute to the class. From the docs:
When Django processes this model, it identifies that it has a ManyToManyField on itself, and as a result, it doesn’t add a person_set attribute to the Person class. Instead, the ManyToManyField is assumed to be symmetrical – that is, if I am your friend, then you are my friend.
So you can add symmetrical=False to the field:
employers = models.ManyToManyField('self', blank=True, related_name='employees', symmetrical=False)
person.employees.all() # will work now
or just use employers attribute:
person.employers.all()
I have two Django models which inherit from a base class:
- Request
- Inquiry
- Analysis
Request has two foreign keys to the built-in User model.
create_user = models.ForeignKey(User, related_name='requests_created')
assign_user = models.ForeignKey(User, related_name='requests_assigned')
For some reason I'm getting the error
Reverse accessor for 'Analysis.assign_user' clashes with reverse accessor for 'Inquiry.assign_user'.
Everything I've read says that setting the related_name should prevent the clash, but I'm still getting the same error. Can anyone think of why this would be happening? Thanks!
The related_name would ensure that the fields were not conflicting with each other, but you have two models, each of which has both of those fields. You need to put the name of the concrete model in each one, which you can do with some special string substitution:
create_user = models.ForeignKey(User, related_name='%(class)s_requests_created')
I recently got a ForeignKey clash in my Django model. I have the need to have two foreign keys (owner, assigned_to) ultimately pointing to the same model (a user).
From what I understand I need a related_name argument to solve that problem. So I did that:
assigned_to = models.ForeignKey(TaskUser, blank=True, null=True, related_name='user_assignment')
and
owner = models.ForeignKey(TaskUser, related_name="user_ownership"
But I'm still getting an error:
tasks.task: Accessor for field 'owner' clashes with related field 'TaskUser.user
_ownership'. Add a related_name argument to the definition for 'owner'.
tasks.task: Reverse query name for field 'owner' clashes with related field 'TaskUser.user_ownership'. Add a related_name argument to the definition for 'owner'.
Why am I still getting this error?
There is one catch, owner is in a super class (BaseWidget) and assigned_to is in a sub class (Task). Are there issues with using related_name in an inheritance relationship? Do I need to just override the inheritance of owner and redefine related_name in the sub class instead? I'd appreciate any help!
If you have ForeignKey relationships in an abstract base class every class inheriting from it will have this relationship. As a result of this you must not 'hardcode' its related_name, because all sub classes will try to create the same accessor on the realted class (TaskUser in this case).
You should better do something like:
owner = models.ForeignKey(TaskUser, related_name="%(app_label)s_%(class)s_ownership")
See the django docs on this.
If you are using related_name in abstract base class you need to use a '%(app_label)s' and '%(class)s' in it.
Its mentioned in django doc
Be careful with related_name