Associations between 3 models

Rails
1

Hi,

I have this requirement where in the association needs to be
between 3 models.

I want a User to select a Gift to be sent to a particular
Destinations.
For this I created a Model RequestForGift with foreign keys
( belongs_to ) to the User, Gift and Destination.Then I added the
has_many relationships to each of the 3 Models to the other 2, but
some how the usage of the association through the models is not
comforting

class User < ActiveRecord::Base
has_many :RequestForGifts
has_many :Gifts, :through => :RequestForGifts, :uniq => true
has_many :Destinations, :through => :RequestForGifts, :uniq => true
end

class Gift < ActiveRecord::Base
has_many :RequestForGifts
has_many :Users, :through => :RequestForGifts, :uniq => true
has_many :Destinations, :through => :RequestForGifts, :uniq => true
end

class Destination < ActiveRecord::Base
has_many :RequestForGifts
has_many :Users, :through => :RequestForGifts, :uniq => true
has_many :Destinations, :through => :RequestForGifts, :uniq => true
end

class RequestForGift < ActiveRecord::Base
belongs_to :Gift
belongs_to :User
belongs_to :Destination
end

i.e. I am not able to find All the Gifts sent by a User to New Delhi,
India

Basically i am looking for some code like

  1. User.find(params[:id]).gifts.select( |g| { g.destination.name =
    ‘New Delhi, India’ } )

I know that the same can be achieved by using the RequestForGift
directly :-

a) destination_id = Destination.find_by_name ( :first, ‘New Delhi,
India’ )
RequestForGift.find ( :all, :conditions => [ ‘destination_id = ? and
user_id = ?’, destination_id, params[:id] ] ).collect ( |r| r.gift )
or
b) User.find(params[:id]).requestForGifts.select ( |rfg|
rfg.destination.name = ‘NewDelhi, India’ ).collect ( |rfg| rfg.gift )

But using query a or b would mean that the :has_many :through
requirements that I have written will be of no use, since they are
only double associative - The association assumes that only 2 models
are involved in the association.

I am facing such models at a really high frequency, and a code
fragment like query 1, I believe would really mprove
understandability, of the code since the association table is never
involved in the ruby code.

Can someone help me out with the model, or if it is even possible with
the current associations that RoR supports?

Thanks,
Abhin H

2

On Dec 16, 3:04 pm, Abhin H [email protected] wrote:

Hi,

I have this requirement where in the association needs to be
between 3 models.

I want a User to select a Gift to be sent to a particular
Destinations.
For this I created a Model RequestForGift

That name will be awkward, since the proper English plural would be
“RequestsForGifts”. Life will be easier if you change the model name
to “GiftRequest” and name the table “gift_requests”.

with foreign keys
( belongs_to ) to the User, Gift and Destination.Then I added the
has_many relationships to each of the 3 Models to the other 2, but
some how the usage of the association through the models is not
comforting

class User < ActiveRecord::Base
has_many :RequestForGifts

Relationship names should be in lower case with underscores. If you
change the model name to “GiftRequest”, then you would have:
has_many :gift_requests
has_many :destinations, :through => :gift_requests, :uniq => true

There may be similar problems with your table definitions. I would
write:
class GiftRequest < ActiveRecord::Base
:belongs_to :user
:belongs_to :gift
:belongs_to :destination
…
end

and in the migration I would have:

add_table :gift_requests do |t|
t.add_column :user_id, :integer, :null => false
t.add_column :gift_id, …
t.add_column :destination_id, …
end

3

If it’s any consolation, the trend these days is to make association
tables into their own models, so perhaps using GiftRequest isn’t so
“dirty,” after all.

  1. User.find(params[:id]).gifts.select( |g| { g.destination.name = ‘New Delhi, India’ } )

I don’t think your desired interface would make logical sense in any
framework, because a gift of a user has more than one destination. I’m
not sure if there could be an interface that would start from
User.gifts.

So I’d just bite the bullet, treat GiftRequest as a first-class
object, and query it directly.

///ark