Question

I’m having a problem writing a scope to return records where ALL has_many associations match a criteria.

I have these models:

class Product
  has_many :listings
end

class Listing
  belongs_to :product
  belongs_to :issue
end

class Issue
  has_many :listings
end

Basically a product can be listed in several different issues. I want to be able to get all products that have NO listings in a particular issue. So far I have this scope in my Product model:

scope :not_listed_in, lambda { |issue|
  joins(:listings)
    .where("listings.issue_id != ?", issue.id)
}

This doesn’t work since it will find any products where at least one listing is not in the issue. I need some way of asking for all products that have NO listings within a particular issue.

Answer 1

Assuming you’re using ActiveRecord, you can achieve this by finding all products and removing products in the issue. That would normally result in an array, so in the code below, I did one additional database query to have it return a scoped result so you can cascade other “where” clauses to the result.

class Product < ActiveRecord::Base
  has_many :listings
  scope :not_listed_in, lambda { |issue|
    id_list = Product.pluck(:id) - issue.products.pluck(:id)
    Product.where(id:id_list)
  }
end

class Listing < ActiveRecord::Base
  belongs_to :product
  belongs_to :issue
end

class Issue < ActiveRecord::Base
  has_many :listings
  has_many :products, through: :listings
end