admin doesn't seem to work with intermediate join table for manytomany

[bryandunbar]

I have many to many that uses through and specifies a join table. The admin integration for Polymorphic inlines doesn't seem to function. Is this a known issue, or am I missing something?

[diogosimao]

Do you get error:

'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'

...?

[JohananOppongAmoateng]

This is not a bug.
django-polymorphic does not support automatic admin inlines for ManyToManyField with a custom through model.

Django admin only works with explicit inlines on the through model, and polymorphic admin doesn’t add extra support on top of that.

What to do:

• Define a normal TabularInline / StackedInline for the through model
• Attach it manually to the PolymorphicChildModelAdmin

[JohananOppongAmoateng]

@bckohan I think this can be made into a discussion too

[bckohan]

@bryandunbar @diogosimao This works, but you have to inherit an inline admin class for the through model from StackedPolymorphicInline:

class A(PolymorphicModel):
    ...

class B(A):
    ...

class AThrough(PolymorphicModel):
    ...

class BThrough(AThrough):  # through models can be polymorphic too!
    ...

class YourModel(models.Model):

    m2m_field = models.ManyToManyField(A, through=AThrough, related_name="my_models", blank=True)

In your admin:

import polymorphic.admin.StackedPolymorphicInline, PolymorphicInlineSupportMixin

class AThroughMembership(StackedPolymorphicInline):

    model = AThrough
    extra = 1

    class AThroughInline(StackedPolymorphicInline.Child):
        ...

        model = AThrough

    class BThroughInline(StackedPolymorphicInline.Child):
        ...

        model = BThrough

    child_inlines = (AThroughInline, BThroughInline)


@register(YourModel)
class YourModelAdmin(PolymorphicInlineSupportMixin, PolymorphicChildModelAdmin):

    inlines = (AThroughMembership,)

You can see a full working example in the tests added in #807