rails has_many :through has_many :through

Question

I'm wondering to what extent I can use associations in Rails. Take into consideration the following:

class User < ActiveRecord::Base
    has_one :provider
    has_many :businesses, :through => :provider
end

class Provider < ActiveRecord::Base
    has_many :businesses
    has_many :bids, :through => :businesses
    belongs_to :user
end

class Business < ActiveRecord::Base
    has_many :bids
    belongs_to :provider
end

class Bid < ActiveRecord::Base
    belongs_to :business
end

I am able to set up these nifty shortcuts like User.businesses and Provider.bids but what about doing something like User.bids? Is it possible to associate an association, so to speak?

Answer 1

Although it is a very useful thing to have, you can't has_many :through a has_many :through relationship. This is a limitation of the join engine.

The alternatives are either to use a clever sub-select, or in this case a sub-sub select, or to deliberately denormalize the tables enough to reduce the join depth.

For example, since a Business is defined within the context of a Provider, it stands to reason that any Bid elements are also assigned, indirectly, to a Provider. Building a direct association between Bid and Provider would make querying bids directly easy.

Answer 2

This is entirely possible, but needs a little extra work. The following model definitions used in conjunction with the nested_has_many plugin you can fetch all bids belonging to a user with just @user.bids

class User < ActiveRecord::Base
    has_one :provider
    has_many :businesses, :through => :provider
    has_many :bids, :through => :businesses
end

class Provider < ActiveRecord::Base
    has_many :businesses
    has_many :bids, :through => :businesses
    belongs_to :user
end

class Business < ActiveRecord::Base
    has_many :bids
    belongs_to :provider
end

class Bid < ActiveRecord::Base
    belongs_to :business
end

However getting a user from a bid will take a more work.

Answer 3

There's nothing stopping you doing something like this afaik:

class User < ActiveRecord::Base
    has_one :provider
    has_many :businesses, :through => :provider

    def bids
        user_bids = []
        businesses.each |business| do
            user_bids += business.bids
        end
        user_bids
    end
end

class Provider < ActiveRecord::Base
    has_many :businesses
    has_many :bids, :through => :businesses
    belongs_to :user
end

class Business < ActiveRecord::Base
    has_many :bids
    belongs_to :provider
end

class Bid < ActiveRecord::Base
    belongs_to :business
end

Then calling @user.bids should produce the desired result, you can also cache the bids and do other fancy stuff if you want.

Answer 4

If you just want to fetch the records, why not use use #delegate? It works just fine, at least in the scenario you've described.

class User < ActiveRecord::Base
    has_one :provider
    delegates :bids, :to => :provider
end

class Provider < ActiveRecord::Base
    has_many :businesses
    has_many :bids, :through => :businesses
    belongs_to :user
end

class Business < ActiveRecord::Base
    has_many :bids
    belongs_to :provider
end

class Bid < ActiveRecord::Base
    belongs_to :business
end

Although in my not-so-humble-opinion you should just chain the methods because it's more straightforward, and you're no longer achieving the performance boost unless you go with some crazy custom SQL as tadman says.