Writing “not in” sql query using AREL

Question

I have a scope defined as follows:

scope :ignore_unavailable, lambda { 
  where([ "Item.id NOT IN (SELECT id FROM Cars WHERE Cars.status = 'NA'" ])
}

Currently its using hardcoded tables names. How can I improve it using frameworks like Arel ? Will appreciate any help here.

I am on Rails 3.2

Answer 1

If you are using rails 4, one way would be

scope :ignore_unavailable, lambda {
  where.not(id: Car.where(:status => "NA").pluck(:id))
}

For rails 3

scope :ignore_unavailable, lambda {
  where("id not in (?)", Car.where(:status => "NA").pluck(:id))
}

Answer 2

Since the task description asks for an answer using AREL, I present following:

class Car
  scope :available, -> { where(arel_table[:status].not_in(['NA'])) }
end

class Item
  scope :available, -> { where(:id => Car.available) }
end

The sql should be something like the following:

SELECT [items].*
FROM [items]
WHERE [item].[id] IN (
    SELECT [cars].[id]
    FROM [cars]
    WHERE [car].[status] NOT IN ('NA')
  )

Obviously, rails 4 has the not scope, so this is a solution for rails 3.

The above code has two benefits:

• It performs a single query
• The table columns are correctly namespaced (unlike when using raw sql)