Question

I have a data model something like this:

# columns include collection_item_id, collection_id, item_id, position, etc class CollectionItem < ActiveRecord::Base   self.primary_key = 'collection_item_id'   belongs_to :collection   belongs_to :item end  class Item < ActiveRecord::Base   has_many :collection_items   has_many :collections, :through => :collection_items, :source => :collection end  class Collection < ActiveRecord::Base   has_many :collection_items, :order => :position   has_many :items, :through => :collection_items, :source => :item, :order => :position end 

An Item can appear in multiple collections and also more than once in the same collection at different positions.

I’m trying to create a helper method that creates a menu containing every item in every collection. I want to use the collection_item_id to keep track of the currently selected item between requests, but I can’t access any attributes of the join model via the Item class.

def helper_method( collection_id )   colls = Collection.find :all   colls.each do |coll|     coll.items.each do |item| # !!! FAILS HERE ( undefined method `collection_item_id' )       do_something_with( item.collection_item_id )     end   end end 

I tried this as well but it also fails with ( undefined method `collection_item’ )

do_something_with( item.collection_item.collection_item_id ) 

Edit: thanks to serioys sam for pointing out that the above is obviously wrong

I have also tried to access other attributes in the join model, like this:

do_something_with( item.position ) 

and:

do_something_with( item.collection_item.position ) 

Edit: thanks to serioys sam for pointing out that the above is obviously wrong

but they also fail.

Can anyone advise me how to proceed with this?

Edit: ——————–>

I found from online documentation that using has_and_belongs_to_many will attach the join table attributes to the retreived items, but apparently it is deprecated. I haven’t tried it yet.

Currently I am working on amending my Collection model like this:

class Collection < ActiveRecord::Base   has_many :collection_items, :order => :position, :include => :item   ... end 

and changing the helper to use coll.collection_items instead of coll.items

Edit: ——————–>

I’ve changed my helper to work as above and it works fine – (thankyou sam)

It’s made a mess of my code – because of other factors not detailed here – but nothing that an hour or two of re-factoring wont sort out.

Answer 1

In your example you have defined in Item model relationship as has_many for collection_items and collections the generated association method is collection_items and collections respectively both of them returns an array so the way you are trying to access here is wrong. this is primarily case of mant to many relationship. just check this Asscociation Documentation for further reference.