Multiple joins with AND where conditions - sql

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
}