Two columns must not equal each other in Rails

Try it like this:

class Friendship < ActiveRecord::Base
  validate :cannot_add_self

  private

  def cannot_add_self
    errors.add(:user2_id, 'You cannot add yourself as a friend.') if user1_id == user2_id
  end
end

if :user1_id == :user2_id

That's always gonna be false - you're comparing the symbols. It's the same as writing if "user1_id" == "user2_id".

You should write it as if user1_id == user2_id to compare the values of the columns.