在轨道上与红宝石建立对称关系的最佳方法是什么？

Question

I am looking for an elegant way to model a symmetric relation (like friendship between users) in rails (though this may be more of a general database question). I know of several approaches, but none seem elegant:

1. Create a user_user relation table with foreign keys of the users. I don't like it since it is essentially an asymmetric solution. I saw an approach that forces the user with the lower id into the first column and the other into the second column, and query the relation from both ends using union. This feels like a hack.

2. Create a friendship table, where each row represent a relation, and:

   • For one-to-one: add a column to the users table to reference the relation
   • For many-to-many: add a user_friendship table that will hold a foreign key to the users and friendships tables.

   The problem with this approach is that is seems cumbersome and also that nothing enforces that the friendship will be shared exactly by 2 users.

I hope that there's a more elegant solution than these. any ideas?

Answer 1

I would create a model Friendship that holds the foreign keys of the users who are friends you can make a validation which ensures that there is only one friendship with the same users simply by doing something like:

validates :friend_id, :uniqueness => { :scope => :user_id }

In my opinion this is a very elegant solution which provides a lot of flexibillity. Checkout the has_one :through association in the rails guide A Guide to Active Record Associations if you want to model something like a friendship.

Of course there is also the has_and_belongs_to_many association. But it does not provide as much flexibillity as if you create a model and thus usually is not such a good idea.