Can't define :joins conditions in has_many relationship? - activerecord

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.