Can't define :joins conditions in has_many relationship?

Question

I have a relationship table :

create_table "animal_friends", :force => true do |t|
    t.integer  "animal_id"
    t.integer  "animal_friend_id"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.integer  "status_id",        :default => 1
  end

linking animals to others. Best way to retreive associations in SQL is :

SELECT animals.* 
from animals join animal_friends as af 
  on animals.id = 
    case when af.animal_id = #{id} then af.animal_friend_id else af.animal_id end 
WHERE #{id} in (af.animal_id, af.animal_friend_id)

And I can't find a way to create a proper has_many relation in rails with this. Apparently, there's no way to provide joining conditions for has_many.

I'm currently using a finder_sql :

has_many :friends, :class_name => "Animal", :finder_sql => 'SELECT animals.* from animals join animal_friends as af on animals.id = case when af.animal_id = #{id} then af.animal_friend_id else af.animal_id end ' +
 'WHERE #{id} in (af.animal_id, af.animal_friend_id) and status_id = #{Status::CONFIRMED.id}'  

but this method has the great disadvantage of breaking activerecord magic. For instance :

@animal.friends.first 

will execute the finder_sql without limit, fetching thousands of rows, then taking the first of the array (and loosing several precious seconds / req).

I guess it's a missing feature from AR, but I'd like to be sure first :) Thanks

Answer 1

You could solve this on the database level with a view, which would be the correct method anyway.

CREATE VIEW with_inverse_animal_friends (
  SELECT id,
         animal_id,
         animal_friend_id,
         created_at,
         updated_at,
         status_id
    FROM animal_friends
   UNION
  SELECT id,
         animal_friend_id AS animal_id,
         animal_id AS animal_friend_id,
         created_at,
         updated_at,
         status_id
    FROM animal_friends
)

If you dont want to have double entries for friends with relations both ways you could do this:

CREATE VIEW unique_animal_friends (
  SELECT MIN(id), animal_id, animal_friend_id, MIN(created_at), MAX(updated_at), MIN(status_id)
    FROM
   (SELECT id,
           animal_id,
           animal_friend_id,
           created_at,
           updated_at,
           status_id
      FROM animal_friends
     UNION
    SELECT id,
           animal_friend_id AS animal_id,
           animal_id AS animal_friend_id,
           created_at,
           updated_at,
           status_id
      FROM animal_friends) AS all_animal_friends
  GROUP BY animal_id, animal_friend_id
)

You would need a way to decide which status_id to use in case there are two conflicting ones. I chose MIN(status_id) but that is probably not what you want.

In Rails you can do this now:

class Animal < ActiveRecord::Base
  has_many :unique_animal_friends
  has_many :friends, :through => :unique_animal_friends
end

class UniqueAnimalFriend < ActiveRecord::Base
  belongs_to :animal
  belongs_to :friend, :class_name => "Animal"
end

This is out of my head and not tested. Also, you might need some plugin for view handling in rails (like "redhillonrails-core").

Answer 2

There is a plugin that does what you want.

There is a post about it here.

There is an alternative here.

Both allow you do to joining conditions and are just using lazy initialization. So you can use dynamic conditions. I find the former prettier, but you can use the latter if you don't want to install plugins.

Answer 3

The two ways to create many to many relationships in active record are has_and_belongs_to_many and has_many :through. This site critiques the differences between the two. You don't have to write any SQL using these methods.