Question

I’ve been stuck on this all day. I have a setup like the one below. I’m trying to define friends using the group_memberships association.

class User < ActiveRecord::Base   has_many :group_memberships   has_many :groups, :through => :group_memberships   has_many :friends # what goes here? << end  class GroupMembership < ActiveRecord::Base   belongs_to :user   belongs_to :role   belongs_to :group end  class Role < ActiveRecord::Base   has_many :group_memberships end  class Group < ActiveRecord::Base   has_many :group_memberships   has_many :users, :through > :group_memberships end 

I’d like to do this without creating a join table for friends, unless it’s completely crazy to do it without.

The group_membership table contains user_id and group_id linking one user to one group.

I’d trying to get

@user.friends 

to return users with common group_memberships using the group_id.

has_many :friends, :through => :group_memberships, :source => :group 

Nothing I’ve tried works, but I’ll chalk that up to my complete misunderstanding of the above code.

Answer 1

Unfortunately Rails doesn’t let you nest has_many’s more than 2 deep.. Forgetting about naming it friends for a moment (let’s call it users instead), this would theoretically be what you’d want:

has_many :group_memberships has_many :groups, :through => :group_memberships has_many :users, :through => groups 

Except that this doesn’t work. If you try it you’ll see this not-so-helpful error message which comes from this bit of code, specifically source_reflection.options[:through].nil?. That is, the through isn’t allowed to have a through itself.

Instead, you may want to do something like this:

Solution 1

class User < ActiveRecord::Base   has_many :group_memberships   has_many :groups, :through => :group_memberships    def friends     groups.with_users.map(&:users).flatten.uniq.reject{|u| u == self}   end end  class Group < ActiveRecord::Base   has_many :group_memberships   has_many :users, :through => :group_memberships    named_scope :with_users, :include => :users end 

Solution 2

Use the nested_has_many_through plugin that Radar mentioned. It looks like at least one fork of it on github has been updated to work on the latest Rails.

Solution 3 (just for kicks)

or, just for kicks, you could do it with one big SQL query:

class User < ActiveRecord::Base   has_many :group_memberships   has_many :groups, :through => :group_memberships    def friends     sql = <<-SQL       SELECT users.* FROM users, (         SELECT DISTINCT gm2.user_id AS user_id         FROM group_memberships gm, groups g, group_memberships gm2         WHERE gm.user_id = ? AND g.id = gm.group_id AND gm2.group_id = g.id AND gm2.user_id != ?       ) AS user_ids       WHERE users.id = user_ids.user_id     SQL     User.find_by_sql([sql, id, id])   end end