Type annotation for InlineModelAdmin with ManyToMany through

[H4rryK4ne]

I have a the following models

# models.py
from django.db.models import ManyToManyField, Model
 

class A(Model):
  class Meta:
    pass


class B(Model):
  a_models = ManyToManyField(to=A, related_name="b_models")
  class Meta:
    pass

and in the Admin I want to use InlineAdminModel to show the B also in A

# admin.py
from my_app.models import A, B
from django.contrib.admin import (
    ModelAdmin,
    StackedInline,
    register,
)


class BInline(StackedInline[A, B]):
    model = B.a_models.through
    extra = 0


@register(A)
class AAdmin(ModelAdmin[A]):
    inlines = [BInline]


@register(B)
class BAdmin(ModelAdmin[B]):
    pass

Mypy complains now:

my_app/admin.py:10: error: Incompatible types in assignment (expression has type "type[B_a_models]", base class "InlineModelAdmin" defined the
type as "type[A]")  [assignment]
        model = B.a_models.through

How do I create a type annotation B_a_models?

[flaeppe]

If you want the type for the implicit through model you'll need to type it out. And once you've typed it out, you might as well use it during runtime, though that's probably not a requirement. But here's the updated models that uses the through model during runtime:

# models.py
-from django.db.models import ManyToManyField, Model
+from django.db.models import ForeignKey, ManyToManyField, Model, UniqueConstraint
 

class A(Model):
  class Meta:
    pass


+class AB(Model):
+    a = ForeignKey(A, on_delete=models.CASCADE)
+    b = ForeignKey("app.B", on_delete=models.CASCADE)
+    
+    class Meta:
+        constraints = [UniqueConstraint(fields=["a", "b"], name="a_b_unq")]
+
+
class B(Model):
-  a_models = ManyToManyField(to=A, related_name="b_models")
+  a_models = ManyToManyField(to=A, through=AB, related_name="b_models")
  class Meta:
    pass