Django migrations - how to make it forget?

Question

I've been sketching out a new Django app with the runserver dev server running in a background window to tracking network wiring, and briefly had this in my model:

class Interface(models.Model):
    name = models.CharField(max_length=200)
    # (blah)

class Connection(models.Model):
    interface_from = models.ForeignKey(Interface, related_name="connections")
    interface_to = models.ForeignKey(Interface, related_name="connections")
    source = models.CharField(max_length=32)

Before I realised I couldn't have the same related_name for two fields. I guess I will need to write something special to find all connections related to an interface, since they might be the 'to' or 'from' end of the connection (be interested in any insight on a nicer way to do that - like a "Set" field)

At this point, I hadn't done a makemigrations, but on stopping the server, and making my migrations, I get:

ERRORS:
autodoc.Connection.interface_from: (fields.E304) Reverse accessor for 'Connection.interface_from' clashes with reverse accessor for 'Connection.interface_to'.
HINT: Add or change a related_name argument to the definition for 'Connection.interface_from' or 'Connection.interface_to'.

Even though there isn't a clash anymore. I don't see a migrations directory anywhere - this was the initial pass at the models - so where is the memory of this error coming from after a restart of the dev server?

Edit: To make it clearer, my Connection model now looks like:

class Connection(models.Model):
    interface_from = models.ForeignKey(Interface)
    interface_to = models.ForeignKey(Interface)
    source = models.CharField(max_length=32)

What does your updated `Connection` model look like? – Josh Kelley Sep 24 '15 at 14:41 — Josh Kelley, Sep 24 '15 at 14:41
change the related_name, can't be equal – Paulo Pessoa Sep 24 '15 at 14:42 — Paulo Pessoa, Sep 24 '15 at 14:42
@JoshKelley - updated the question for you – AnotherHowie Sep 24 '15 at 15:10 — AnotherHowie, Sep 24 '15 at 15:10
@PauloPessoa - I know, that's why I removed it again! :-) – AnotherHowie Sep 24 '15 at 15:11 — AnotherHowie, Sep 24 '15 at 15:11
try to add the related_name in only one field – Paulo Pessoa Sep 24 '15 at 15:18 — Paulo Pessoa, Sep 24 '15 at 15:18

score 2 · Answer 1 · answered Sep 24 '15 at 14:43

2

In case you don't need a backwards relation add related_name='+' to your field definition. From the doc:

user = models.ForeignKey(User, related_name='+')

answered Sep 24 '15 at 14:43

Ivan

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Without this, I'll get (mostly useless but not harmful) Interface.interface_from_set and Interface.interface_to_set right? Good to know that can be disabled, once the original problem is resolved... – AnotherHowie Sep 24 '15 at 15:15

score 1 · Accepted Answer · edited May 23 '17 at 11:44

In your first example:

class Connection(models.Model):
    interface_from = models.ForeignKey(Interface, related_name="connections")
    interface_to = models.ForeignKey(Interface, related_name="connections")

You're telling Django to create two different connections attributes on Interface for the backwards relationships back to Connection, which obviously doesn't work.

In your second example:

class Connection(models.Model):
    interface_from = models.ForeignKey(Interface)
    interface_to = models.ForeignKey(Interface)

You're telling Django to use its default connections_set name for two different attributes for the backwards relationships back to Connection, which also doesn't work.

The fix is to use related_name='+' (as @Ivan said) to disable the backwards relationships completely or two explicitly provide two different related_name attributes so that the backwards relationship attributes' names don't clash.

D'oh - I was thinking it would use the field name not the model name for the backwards relationship for some reason. Thanks for the explanation! — AnotherHowie, Sep 24 '15 at 15:37

Django migrations - how to make it forget?

2 Answers2

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