Add validation for clashing ManyToManyField.db_table names

[strelnikovdmitrij]

Originally from documentation.
ManyToManyField.db_table
The name of the table to create for storing the many-to-many data. If this is not provided, Django will assume a default name based upon the names of the two tables being joined.

It is a little bit unclear, cause if db_table is not provided, Django will create table with name based on the name of the model (table that has the ManyToManyField), and field name that contain this ManyToManyField.

class Product(models.Model):
 color = models.ManyToManyField('ProductColor', null=True, blank=True)
 class Meta:
  db_table = 'eshop_product'

class ProductColor(models.Model):
 name = ...
 class Meta:
  db_table = 'eshop_product_color'

running syncdb
after the creation of 'eshop_product' table Django will create 'eshop_product_color' table that holds relation between Product and ProductColor. The table for ProductColor will never create.

[Alex Gaynor]

Reformatted, please use preview.

[Russell Keith-Magee]

This is a validation condition that should be caught - the generated m2m table name should recognize the clash with the manually specified db_table.

[coleifer]

This one is pretty hard to validate against, since the m2m_db_table gets set when the ManyToManyField contributes to class, and any other models that may cause conflicts might not be loaded yet.

One way to get this to work is to specify db_table on your ManyToManyField::

from django.db import models

class Product(models.Model):
    color = models.ManyToManyField('ProductColor', db_table='eshop_product_color_rel', 
                                   null=True, blank=True)
    class Meta:
        db_table = 'eshop_product'

class ProductColor(models.Model):
    name = models.CharField(max_length=200)
    class Meta:
        db_table = 'eshop_product_color'

Your table gets created correctly:

charles@charles-laptop:~/tmp/test_m2m_collision$ ./manage.py syncdb
Creating table django_session
Creating table eshop_product_color_rel
Creating table eshop_product
Creating table eshop_product_color

[James Bennett]

If this is a bug it's a validation bug -- since the problem is we're not warning that you've set up two things which each want the same table name -- but it's enough of an edge case that I don't think it needs to be in 1.2.

[Jacob]

Milestone 1.3 deleted

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

​PR

[Tim Graham]

Tests do not run. Please uncheck "Patch needs improvement" when you update it, thanks.

[Berker Peksag]

​https://github.com/django/django/pull/6711

[Tim Graham <timograham@…>]

In 0bce2f1:

Fixed #12810 -- Added a check for clashing ManyToManyField.db_table names.