new object with readonly m2m field through an error in the admin

[benc]

I have a model with m2m field that I want to be readonly.

I want the admin user to be able to see this field but not edit it and I'll populate it in the model save method.

When trying to create a new model instance I'm getting:

instance needs to have a primary key value before a many-to-many relationship can be used.

It should be able to just put an empty string or None in the readonly field.

Thanks

[Karen Tracey]

This is a dupe of #13004.