How to access rails join model attributes when using has_many :through?

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.

Thanks for the useful link. And for pointing out that "item. Collection_item.

Whatever" is obviously wrong. I'll amend my question to reflect that. The gist of my questions is: (how) can I get at the join table attributes via the coll.

Items array? – Noel Walters Mar 12 '09 at 10:54.

Do_something_with( item. Collection_item_id ) This fails because item does not have a collection_item_id member. Do_something_with( item.

Collection_item. Collection_item_id ) This fails because item does not have a collection_item member. Remember that the relation between item and collection_items is a has_many.

So item has collection_items, not just a single item. Also, each collection has a list of collection items. What you want to do is probably this: colls = Collection.

Find :all colls. Each do |coll| coll. Collection_items.

Each do |collection_item| do_something_with( collection_item. Id ) end end A couple of other pieces of advice: Have you read the documentation for has_many :through in the Rails Guides? It is pretty good.

You shouldn't need the :source parameters in the has_many declarations, since you have named your models and associations in a sensible way. 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.

I recommend you stick with has_many :through, because has_and_belongs_to_many is more confusing and doesn't offer any real benefits.

Thanks - If I could accept two answers I would - I did read the documentation for has_many :through and now that I know the answer it seems obvious - too many late nights! – Noel Walters Mar 12 '09 at 12:08.

I was able to get this working for one of my models: class Group :memberships, :source => :user do def with_join proxy_target. Map do |user| proxy_owner = proxy_owner() user.metaclass. Send(:define_method, :membership) do memberships.

Detect {|_| _. Group == proxy_owner} end user end end end end In your case, something like this should work (haven't tested): class Collection :position has_many :items, :through => :collection_items, :source => :item, :order => :position do def with_join proxy_target. Map do |items| proxy_owner = proxy_owner() item.metaclass.

Send(:define_method, :join) do collection_items. Detect {|_| _. Collection == proxy_owner} end item end end end end Now you should be able to access the CollectionItem from an Item as long as you access your items like this (items.

With_join): def helper_method( collection_id ) colls = Collection. Find :all colls. Each do |coll| coll.items.

With_join. Each do |item| do_something_with( item.join. Collection_item_id ) end end end Here is a more general solution that you can use to add this behavior to any has_many :through association: http://github.com/TylerRick/has_many_through_with_join_model class Collection :position has_many :items, :through => :collection_items, :source => :item, :order => :position, :extend => WithJoinModel end.

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