#22397: Issues removing M2M field with explicit through model
----------------------------+------------------------
Reporter: melinath | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7-beta-1
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
----------------------------+------------------------
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')
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/22397>
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