unhashable type: 'RelatedManager' when deleting a model using django admin that has GenericRelation and related_query_name

[Federico Jaramillo Martínez]

Ok, a few things before he report:

• This is my first Django bug report (hope I do well)
• Didn't know how else to call the report, a better summary is appreciated.

The problem:
When using django admin to delete an instance of a model that has a GenericRelation with a related_query_name, an "unhashable type: 'RelatedManager'" error is thrown.

If the related_query_name is removed, the error disappears.

Example structure:

...

class ModelA(models.Model):

    """Model with GenericForeignKey."""

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey()


class ModelB(models.Model):

    """Model with a GenericRelation and related_query_name."""

    model_a = GenericRelation(ComponenteContable, related_query_name='model_a')


class ModelC(models.Model):

    """Model with a GenericRelation and related_query_name."""

    model_c = GenericRelation(ComponenteContable, related_query_name='model_c')


class ModelD(models.Model):

    """Model with a GenericRelation and related_query_name."""

    model_d = GenericRelation(ComponenteContable, related_query_name='model_d')

...

It is not possible to delete a model B, C or D instance in django admin if the have a related_query_name.

The moment you remove related_query_name, that Model's instances can be deleted.

[Federico Jaramillo Martínez]

A bit of debugging shows that when deleting an instance, django admin finds all the instances that are related to it in order to delete them as well.

Problem is the dictionary created to store all the related instances to be deleted uses those same instances as keys in order to recursively find the whole tree.

When GenericRelation has a related_query_name, it generates a dynamic "RelatedManager" class that is not hashable.

This all happens in a NestedObjects class defined in django.contrib.admin.utils

From there, I got lost.

[Tim Graham]

Bisected to aa5ef0d4fc67a95ac2a5103810d0c87d8c547bac. Attaching a regression test for Django's test suite.

[Markus Holtermann]

PR: ​https://github.com/django/django/pull/4885

[Markus Holtermann <info@…>]

In d3d66d4:

Fixed #24940 -- Made model managers hashable

Thanks Federico Jaramillo Martínez for the report and Tim Graham for the
test and review.

[Markus Holtermann <info@…>]

In f64a3de2:

[1.8.x] Fixed #24940 -- Made model managers hashable

Thanks Federico Jaramillo Martínez for the report and Tim Graham for the
test and review.

Backport of d3d66d47222dd8765a20a15fdc754c0ed7635404 from master