Find Products matching ALL Categories (Rails 3.1)

Question

I am struggling with an ActiveRecord query in Rails 3.1.1.

I have 2 models, Product and Category, with a has_and_belongs_to_many from Product to Category (a Product

Answer 1

results = Product.joins(:category_products)
[1,5,8].each do |category|
  results = results.where('category_products.category_id' => category)
end

Answer 2

You can do this with a having clause:

@ids = [1,5,8]
query = Product.select('products.id,products.name').joins(:categories) \
               .where(:categories => {:id => @ids}) \
               .group('products.id, products.name') \
               .having("count(category_products.category_id) = #{@ids.length}")