Question

membership = Membership.where(user_id: 1, group_id: 1).first
# some other code
membership.update_attributes(admin: true)

generates a query

UPDATE 'memberships' SET 'admin' = 1 WHERE 'memberships'.'' IS NULL

which results in an error.

Answer 1

'memberships'.'' IS NULL have to be 'memberships'.'id'=

And there is no id in model I suppose.

Then the right way to update attribute will be something like that

membership = User.find(1).memberships.first
# some other code
membership.update_attributes(admin: true)

UPDATE

Seems adding the id to this model will be the best choice.