基于has_many作用域计数的Rails活动记录作用域

Question

I have Order model and a Container model with a scope as below:

class Order < ActiveRecord::Base
  has_many :containers, inverse_of: :order, dependent: :destroy
end

class Container < ActiveRecord::Base
  scope :full_pickup_ready, -> { where.not(full_pickup_ready_date: nil) }
end

The order model has a field call quantity which represents the quantity of containers the order requires but is not necessarily the size of the containers association as not all container data is entered at the time of order creation.

I would like to have a scope on the Order model based on whether the count of the containers with a full_pickup_ready_date is less than the quantity field of the order.

I know I can use merge on the order model to access the scope on the containers like this:

def self.at_origin
  joins(:containers).merge(Container.full_pickup_ready).uniq
end

but how can I limit the scope to orders where the total number of containers with a full_pickup_ready_date is less that the quantity field on the order?

UPDATE: this is reasonably close, but I don't think using the select is efficient:

includes(:containers).select {|o| o.containers.full_pickup_ready.size < o.quantity }

Answer 1

If you're willing to forgo reuse of the scope on Container , then you should be able to use something like:

# scope on Order
joins(:containers)
  .group("orders.id")
  .having("count(CASE WHEN full_pickup_ready_date THEN 1 END) < orders.quantity")

Answer 2

I think you need to get this SQL query to work for you. So the idea is to get the SQL query right, and then translate it to "Rails".

SQL

If I am correct, this should be the SQL query you want to achieve. Maybe you can try it in your rails db

SELECT orders.* 
FROM orders JOIN containers 
WHERE containers.id = orders.id 
AND ( 
  SELECT COUNT(containers.id) 
  FROM containers 
  WHERE containers.full_pickup_ready_date IS NOT NULL 
) < orders.quantity;

ActiveRecord

If this is the right query, then we can do this using rails

Order.joins(:containers).where("( SELECT COUNT(containers.id) FROM containers WHERE containers.full_pickup_ready_date IS NOT NULL ) < orders.quantity")

This should return an ActiveRecord relation. You could also do this:

sql = %{
  SELECT orders.* 
  FROM orders JOIN containers 
  WHERE containers.id = orders.id 
  AND ( 
    SELECT COUNT(containers.id) 
    FROM containers 
    WHERE containers.full_pickup_ready_date IS NOT NULL 
  ) < orders.quantity;
}.gsub(/\s+/, " ").strip

Order.find_by_sql(sql)

Just add this in a class method (better than scope IMHO) and you are good to go.

So, your Order class should look like this:

class Order < ActiveRecord::Base
  has_many :containers, inverse_of: :order, dependent: :destroy

  def self.gimme_a_query_name
    joins(:containers).where("( SELECT COUNT(containers.id) FROM containers WHERE containers.full_pickup_ready_date IS NOT NULL ) < orders.quantity")
  end

  def self.gimme_another_query_name
    sql = %{
      SELECT orders.* 
      FROM orders JOIN containers 
      WHERE containers.id = orders.id 
      AND ( 
        SELECT COUNT(containers.id) 
        FROM containers 
        WHERE containers.full_pickup_ready_date IS NOT NULL 
      ) < orders.quantity;
    }.gsub(/\s+/, " ").strip
    find_by_sql(sql)
  end
end

I have no way to try this, but it should work with few tweak to get the SQL query right.

I hope this help!