Elegantly selecting attributes from has_many :through join models in Rails

Question

I\'m wondering what the easiest/most elegant way of selecting attributes from join models in has_many :through associations is.

Lets say we have Items, Catalogs, and Cat

Answer 1

You should be able to do @catalog.catalog_item.position if you provide the other end of the association.

class Catalog < ActiveRecord::Base
  belongs_to :catalog_item
end

Now you can do Catalog.first.catalog_item.position.

Answer 2

You can do something like this:

# which is basically same as your "frustrating way" of doing it
@item.catalog_items.find_by_catalogue_id(@item.catalogs.first.id).position

Or you can wrap it into in an instance method of the Item model:

def position_in_first_catalogue
  self.catalog_items.find_by_catalogue_id(self.catalogs.first.id).position
end

and then just call it like this:

@item.position_in_first_catalogue

Answer 3

Why don't You just

@item = Item.find(4)
position = @item.catalog_items.first.position

why do you go through catalogs? It doesn't make any sense to me since you are looking for first ANY catalog!?

Answer 4

Just adding answer so that it might help others

CatalogItem.joins(:item, :catalog).
  where(items: { id: 4 }).pluck(:position).first