id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
22397	Issues removing M2M field with explicit through model	Stephen Burrows	andrewsg	"I get the error below when I try to delete a field that has an explicit through model. It comes down to: for some reason, field.rel.through is a string instead of being resolved to a model (maybe in part because the model has been deleted already?)

{{{
  File ""./manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "".../django/django/core/management/__init__.py"", line 427, in execute_from_command_line
    utility.execute()
  File "".../django/django/core/management/__init__.py"", line 419, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "".../django/django/core/management/base.py"", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "".../django/django/core/management/base.py"", line 337, in execute
    output = self.handle(*args, **options)
  File "".../django/django/core/management/commands/migrate.py"", line 145, in handle
    executor.migrate(targets, plan, fake=options.get(""fake"", False))
  File "".../django/django/db/migrations/executor.py"", line 60, in migrate
    self.apply_migration(migration, fake=fake)
  File "".../django/django/db/migrations/executor.py"", line 94, in apply_migration
    migration.apply(project_state, schema_editor)
  File "".../django/django/db/migrations/migration.py"", line 97, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File "".../django/django/db/migrations/operations/fields.py"", line 83, in database_forwards
    schema_editor.remove_field(from_model, from_model._meta.get_field_by_name(self.name)[0])
  File "".../django/django/db/backends/sqlite3/schema.py"", line 125, in remove_field
    if isinstance(field, ManyToManyField) and field.rel.through._meta.auto_created:
AttributeError: 'str' object has no attribute '_meta'
}}}


If I reverse the order of operations (so that the field is removed *before* the through model is deleted) I get the following error:

{{{
  File ""./manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "".../django/django/core/management/__init__.py"", line 427, in execute_from_command_line
    utility.execute()
  File "".../django/django/core/management/__init__.py"", line 419, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "".../django/django/core/management/base.py"", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "".../django/django/core/management/base.py"", line 337, in execute
    output = self.handle(*args, **options)
  File "".../django/django/core/management/commands/migrate.py"", line 145, in handle
    executor.migrate(targets, plan, fake=options.get(""fake"", False))
  File "".../django/django/db/migrations/executor.py"", line 60, in migrate
    self.apply_migration(migration, fake=fake)
  File "".../django/django/db/migrations/executor.py"", line 94, in apply_migration
    migration.apply(project_state, schema_editor)
  File "".../django/django/db/migrations/migration.py"", line 97, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File "".../django/django/db/migrations/operations/fields.py"", line 83, in database_forwards
    schema_editor.remove_field(from_model, from_model._meta.get_field_by_name(self.name)[0])
  File "".../django/django/db/backends/sqlite3/schema.py"", line 128, in remove_field
    self._remake_table(model, delete_fields=[field])
  File "".../django/django/db/backends/sqlite3/schema.py"", line 65, in _remake_table
    del body[field.name]
KeyError: 'items'
}}}

Model is changing from

{{{
class ItemDiscount(models.Model):
    PERCENT = 'percent'
    FLAT = 'flat'

    TYPE_CHOICES = (
        (PERCENT, _('Percent')),
        (FLAT, _('Flat')),
    )
    item = models.ForeignKey(Item)
    discount = models.ForeignKey('Discount')
    discount_type = models.CharField(max_length=7,
                                     choices=TYPE_CHOICES,
                                     default=PERCENT)
    amount = models.DecimalField(max_digits=5, decimal_places=2,
                                 blank=True, null=True,
                                 validators=[MinValueValidator(0)])


class Discount(models.Model):
    name = models.CharField(max_length=40)
    code = models.CharField(max_length=20)
    items = models.ManyToManyField(Item, through=ItemDiscount)
    available_start = models.DateTimeField()
    available_end = models.DateTimeField()
    event = models.ForeignKey(Event)

    class Meta:
        unique_together = ('code', 'event')
}}}

to

{{{
class Discount(models.Model):
    PERCENT = 'percent'
    FLAT = 'flat'

    TYPE_CHOICES = (
        (PERCENT, _('Percent')),
        (FLAT, _('Flat')),
    )
    name = models.CharField(max_length=40)
    code = models.CharField(max_length=20)
    item_option = models.ForeignKey(ItemOption)
    available_start = models.DateTimeField(blank=True, null=True)
    available_end = models.DateTimeField(blank=True, null=True)
    discount_type = models.CharField(max_length=7,
                                     choices=TYPE_CHOICES,
                                     default=PERCENT)
    amount = models.DecimalField(max_digits=5, decimal_places=2,
                                 blank=True, null=True,
                                 validators=[MinValueValidator(0)])
    event = models.ForeignKey(Event)

    class Meta:
        unique_together = ('code', 'event')
}}}"	Bug	closed	Migrations	dev	Release blocker	fixed		Stephen Burrows loic@…	Accepted	0	0	0	0	0	0