django Cannot set values on a ManyToManyField which specifies an intermediary model. Use Manager instead

If you are allowed to modify class Membership, adding auto_created = True might solve your problem,

class Membership(models.Model): 
    class Meta:
        auto_created = True

In Django 1.7, the error message is changed to "Cannot set values on a ManyToManyField which specifies an intermediary model". The solution is the same.

NOTE: This will remove your intermediate model entirely, and all the additional fields with it.

As seen on:

http://docs.djangoproject.com/en/dev/topics/db/models/#intermediary-manytomany

Unlike normal many-to-many fields, you can't use add, create, or assignment (i.e., beatles.members = [...]) to create relationships

I guess your code trips up on the line "form.save_m2m()", which is unnecessary since you already manually create a membership.

I had a similar error message on a different problem. I post it here just in case it helps others.

I've added a new ManyToManyField on an existing model. This model was used in a ModelForm built with an exclude field.

I fixed the problem by add the new field in the excluded ones.