def invited_by?(user)
      friendship = find_any_friendship_with(user)
      return false if friendship.nil?
      friendship.friendable_id == user.id
    end
def invited_by?(user)
      friendship = find_any_friendship_with(user)
      return false if friendship.nil?
      friendship.friendable_id == user.id
    end
checks if a current user received invitation from given user