Rails validating uniqueness of polymorphic association

Question

I have the following:

class Membership < ApplicationRecord
  belongs_to :member, polymorphic: true
  belongs_to :group, polymorphic: true
end

As you can see my Membership model is doubly polymorphic. I want to validate the uniqueness of :member scoped by :group .

I was hoping the following would work:

validates :member, uniqueness: { scope: :group }

But this causes an error:

NameError: uninitialized constant Membership::Member

Is there a Rails way of doing this, or do I need to write a custom method for it?

Answer 1

validates :member_id, :uniqueness => { :scope => [:member_type, :group_id,:group_type] }

With this, you might also want to make presence of member and group both mandatory, as nil values will lead to duplicates and will fail the uniqueness validation

validates :member_type, :member_id, :presence => true
validates :group_type, :group_id, :presence => true

Answer 2

I used rohan's answer but added another validation for group_id

validates :group_id, uniqueness: { :scope => [:group_type, : member_id,: member_type] }
validates :member_id, uniqueness: { :scope => [:member_type, : group_id,: group_type] }

I also added a DB index

add_index :memberships, [:group_id, :group_type, :member_id, :member_type], unique: true

Answer 3

I ended up with this. ( on: :create allows updating)

validate :group_member_uniqueness, on: :create
def group_member_uniqueness
  if Membership.where(group: group, member: member).exists?
    errors[:base] << 'That has already been added'
  end
end

DB index

add_index :memberships, [:group_id, :group_type, :member_id, :member_type], unique: true

Answer 4

There is no built-in for this, so I did the following:

validate :member_is_unique_for_group

def member_is_unique_for_group
  if group.members.include? member
    errors.add(:member, 'already exists for this group')
  end
end