Opened 5 years ago
Closed 5 years ago
#31263 closed Bug (needsinfo)
remote_field model caching issues in RenameModel migration.
Reported by: | Sean Esterkin | Owned by: | nobody |
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Component: | Migrations | Version: | 2.2 |
Severity: | Normal | Keywords: | migration, caching, remote_field, RenameModel |
Cc: | Triage Stage: | Unreviewed | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
I am encountering a bug when renaming a model with RenameModel. My project is fairly large with multiple applications and many migrations. With the project in its current state, the migrations run correctly. I have added a new migration with some earlier dependencies, which changed the order that some of the migrations ran in, and now one of the Many to Many tables foreign key column is not being renamed correctly.
I have tracked this issue down to the function _populate_directed_relation_graph
within django/db/models/options.py
. This function loops through all of the models and populates a related_objects_graph with each model pointing to a list of related fields.
Related fields are populated here
for f in fields_with_relations: if not isinstance(f.remote_field.model, str): related_objects_graph[f.remote_field.model._meta.concrete_model._meta].append(f)
and accessed here:
related_objects = related_objects_graph[model._meta.concrete_model._meta] model._meta.__dict__['_relation_tree'] = related_objects
The bug seems to be that f.remote_field.model._meta.concrete_model._meta
is not the same for every field pointing to what should be the same model. For example if my code looked like:
class M1(models.Model): field1 = models.BooleanField() class M2(models.Model): m1 = models.ForeignKey(M1, on_delete=models.CASCADE) class M3(models.Model): m1s = models.ManyToManyField(M1, related_name="m3s")
then the populated related_objects
for M1 might look like
defaultdict(<class 'list'>, {<Options for M1>: [<django.db.models.fields.related.ForeignKey: m1>], <Options for M1>: [<django.db.models.fields.related.ManyToManyField: m1s>]}
where each <Options for M1> is slightly different.
This is very likely a caching bug. When I disable the @cached_property (see below) the bug does not occur. Additionally the bug only occurs when I run the migrations start to finish. If I run the migrations one at a time in the same order, this bug does not happen.
For diagnostics, I have found two changes to the Django code that cause this bug to stop occurring. Neither of these are good changes to the Django code.
The first is to change
related_objects_graph[f.remote_field.model._meta.concrete_model._meta].append(f)
to
related_objects_graph[f.remote_field.model._meta.concrete_model._meta.__str__()].append(f)
and
related_objects = related_objects_graph[model._meta.concrete_model._meta]
to
related_objects = related_objects_graph[model._meta.concrete_model._meta.__str__()]
The other fix involved editing the __get__
function on cached_property
in django/utils/functional.py
.
if instance is None: return self res = instance.__dict__[self.name] = self.func(instance) return res
becomes
if instance is None: return self return self.func(instance)
crippling the caching but fixing the caching bug.
Change History (3)
comment:1 by , 5 years ago
Type: | Uncategorized → Bug |
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comment:3 by , 5 years ago
Resolution: | → needsinfo |
---|---|
Status: | new → closed |
Summary: | remote_field model caching issues in RenameModel migration → remote_field model caching issues in RenameModel migration. |
That's likely an issue with
RenameModel.state_forwards
https://github.com/django/django/blob/1712a76b9dfda1ef220395e62ea87079da8c9f6c/django/db/migrations/operations/models.py#L304-L343
Could you set
delay=False
in thereload_models
calls and see if it helpshttps://github.com/django/django/blob/1712a76b9dfda1ef220395e62ea87079da8c9f6c/django/db/migrations/operations/models.py#L339-L343
That should trigger an immediate re-rerender of all
to_reload
models instead of deferring on demand which seems to be the issue here.It's also possible that this is due to
ManyToManyField
auto-created models (when no explicitthrough
is specified like in your example) not being reloaded as it seems we only deal with thethrough is not None
casehttps://github.com/django/django/blob/1712a76b9dfda1ef220395e62ea87079da8c9f6c/django/db/migrations/operations/models.py#L325-L333
In order to address this issue we'll need a way to reproduce it so if you could reduce your large project to a simplified set of apps/migrations that would be greatly appreciated.