[Django] #29314: PostgreSQL error when using a TextField generic object ID field, an integer PK, and GenericRelation

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Apr 10, 2018, 8:13:53 PM4/10/18

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#29314: PostgreSQL error when using a TextField generic object ID field, an integer
PK, and GenericRelation
-------------------------------------+-------------------------------------
Reporter: Kye | Owner: nobody
Russell |
Type: Bug | Status: new
Component: Database | Version: 1.11
layer (models, ORM) |
Severity: Normal | Keywords: orm, postgres
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I'm still unsure if this is a Django bug or incorrect usage on my part, so
please close this ticket if it's the latter.

I have a fairly basic looking Django model with a generic relationship:

{{{
class AttachedMediaItem(models.Model):

# Generic relation to another object.
parent_content_type = models.ForeignKey(
'contenttypes.ContentType',
related_name='attachedmediaitem_parent_set',
on_delete=models.CASCADE)
parent_object_id = models.TextField(db_index=True)
parent_content_object = GenericForeignKey('parent_content_type',
'parent_object_id')
}}}

(irrelevant fields removed)

I inherited this codebase so I cannot fully justify all design decisions,
however I believe parent_object_id is a TextField to support non-integer
PKs on the related object (e.g. UUIDs). This model tends to relate to a
wide variety of other models, so it needs to be very versatile in terms of
what PK types it supports.

This is as-per the Django docs recommendations:
https://docs.djangoproject.com/en/2.0/ref/contrib/contenttypes/#django.contrib.contenttypes.fields.GenericForeignKey

Now, this model:

{{{
class UnitType(models.Model):

media = GenericRelation('media.AttachedMediaItem',
content_type_field='parent_content_type',
object_id_field='parent_object_id')
}}}

(irrelevant fields removed).

Note that I'm leaving the PK generation up to Django, meaning I'll get an
integer PK for this model.

Now, if I run this

{{{
UnitType.objects.filter(media__isnull=True)
}}}

An SQL error manages to bubble through the ORM:

{{{
ProgrammingError: operator does not exist: integer = text
LINE 1: ...a_attachedmediaitem" ON ("products_unittype"."id" = "media_a...
^
HINT: No operator matches the given name and argument type(s). You might
need to add explicit type casts.
}}}

My understanding is that this is due to the difference in PK fields.

I believe I have followed the prescribed best practice as per the Django
documentation (relevant section linked above). I feel that at the very
least this shouldn't result in a somewhat cryptic PostgreSQL error.

Is this a genuine Django bug, or misuse on my part?

--
Ticket URL: <https://code.djangoproject.com/ticket/29314>
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Django

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Apr 10, 2018, 10:49:30 PM4/10/18

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#29314: PostgreSQL error when using a TextField generic object ID field, an integer
PK, and GenericRelation
-------------------------------------+-------------------------------------

Keywords: orm, postgres | Triage Stage:

| Unreviewed
Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Tim Graham):

* status: new => closed
* component: Database layer (models, ORM) => contrib.contenttypes
* resolution: => duplicate