Django GenericRelation still does not enable reverse querying from GenericForeignKey

Question

In my Django project I have a model named Value that has a GenericForeignKey as such:

class Value(models.Model):
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE, blank=True, null=True)
    val_id = models.PositiveIntegerField(blank=True, null=True)
    data_obj = GenericForeignKey('content_type', 'val_id')

Value is meant to be a sort of polymorphic table that uses ContentType and GenericForeignKey to be able to point to an arbitrary table that contains the actual data. For example, there is a model Int that a Value can point to:

class Int(models.Model):
    data = models.IntegerField(blank=True, null=True)

So, after creating an instance of Int:

myint = Int.objects.create(data=1)

I can create a Value that points to it:

myval = Value.objects.create(data_obj=myint)

There are other similar models that a Value would point to, such as UInt, String, and Float; they all only have the one field data. However, I was wondering how to query/filter through instances of Value based on the data contained by the model pointed to by the field data_obj. I.e., I want to be able to do something like this:

Value.objects.filter(data_obj__data=1)

But this results in an error:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/django/db/models/manager.py", line 122, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 790, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 808, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1243, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1269, in _add_q
    allow_joins=allow_joins, split_subq=split_subq,
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1149, in build_filter
    lookups, parts, reffed_expression = self.solve_lookup_type(arg)
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1035, in solve_lookup_type
    _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1316, in names_to_path
    "adding a GenericRelation." % name
FieldError: Field 'data_obj' does not generate an automatic reverse relation and therefore cannot be used for reverse querying. If it is a GenericForeignKey, consider adding a GenericRelation.

The problem is that even though I've updated each data model to have a GenericRelation field back to the Value model:

class Int(models.Model):
    data = models.IntegerField(blank=True, null=True)
    val_obj = GenericRelation(Value, object_id_field="val_id")

class UInt(models.Model):
    data = models.PositiveIntegerField(blank=True, null=True)
    val_obj = GenericRelation(Value, object_id_field="val_id")

class String(models.Model):
    data = models.CharField(max_length=2048)
    val_obj = GenericRelation(Value, object_id_field="val_id")

# etc.

I still can't get reverse querying to work and get the same error from above. What am I doing wrong here that adding GenericRelation still does not allow for reverse querying or seem to change anything at all? I would definitely like the reverse querying to work because the project has a filtering script that would be much easier to make work on Values if I could only just reverse query successfully.

UPDATE: I figured out how to not get a FieldError, but still cannot get reverse querying to work the way I want. What I did was add a related_query_name to each of the Int, UInt, Float, etc. data models' GenericRelation fields:

val_obj = GenericRelation(Value, object_id_field="val_id", related_query_name="val")

And according to the Django documentation, I believe I would then reverse query like this:

Value.objects.filter(val__data=1)

Yet, even though I definitely have data models whose data field contains the int 1, this and any other reverse querying returns an empty list. Is there something else I'm doing wrong/missing?

Answer 1

I had the same problem and found I had to define a unique related_query_name for each related model, as described here:

https://stackoverflow.com/a/36166644/1143466

For example:

class Int(models.Model):
    data = models.IntegerField(blank=True, null=True)
    val_obj = GenericRelation(Value, object_id_field="val_id",
                              related_query_name="int")

class UInt(models.Model):
    data = models.PositiveIntegerField(blank=True, null=True)
    val_obj = GenericRelation(Value, object_id_field="val_id",
                              related_query_name="uint")

class String(models.Model):
    data = models.CharField(max_length=2048)
    val_obj = GenericRelation(Value, object_id_field="val_id",
                              related_query_name="string")

Then you would create a filter by querying each model separately, e.g.:

from django.db.models import Q

val = 1
val_filter = Q(int__data=val) | Q(uint__data=val) | Q(string__data=val)
filtered_values = Value.objects.filter(val_filter)