Question

I have a simple tagging system of items with the following structure:

 class Item < ActiveRecord::Base
     has_many :taggings, :dependent => :destroy
     has_many :tags, :through => :taggings
 end

 class Tagging < ActiveRecord::Base
     belongs_to :tag
     belongs_to :item
 end

 class Tag < ActiveRecord::Base
     has_many :taggings, :dependent => :destroy
 end

I’d like to add a scope to the Item class to be able to get all the items with a given set of tags (all tags in the set included)

So I tried the following scope:

 scope :tag_context, lambda { |context| (context.empty? ? all :
       joins(:taggings).where(:taggings => {tag_id => context.collect(&:id)})
       )
  }

where context is an Array of Tag objects.

The point is that this scope yield the following sql:

SELECT items.* FROM items INNER JOIN taggings ON taggings.item_id = items.id 
                          WHERE taggings.tag_id IN (1,2)

assuming context contains tag 1 and 2.

I would like to get items that are tagged by tag 1 AND tag 2.
So I assume, something like:

SELECT items.* FROM items INNER JOIN taggings as t1 ON t1.item_id = items.id 
                          INNER JOIN taggings as t2 ON t2.item_id = items.id 
                          WHERE t1.tag_id = 1 AND t2.tag_id = 2

How should I proceed to translate it in a Rails scope?
I need a scope in order to be able to chain the various scopes on the Item class.

Thanks for your help!

Answer 1

You can try building the scope dynamically like this (and you don’t need an .empty? check with inject):

 scope :tag_context, lambda { |context| 
      context.collect(&:id).inject(self) do |m, id| 
        m.joins("INNER JOIN taggings as t#{id} ON t#{id}.item_id = items.id").
        where("t#{id}.tag_id = ?", id)
      end
 }