Use scope of a belongs_to association to scope in the first model - Ruby on Rails

Question

Problem

Lets say I have two Models with a has_many-belongs_to relation. The has_many has a scope defined and an integer attribute named grade.

class Parent < ApplicationRecord
  has_many :children
  scope :great, -> (min_grade) {where("grade > :grade", grade: min_grade)}
end

class Child < ApplicationRecord
  belongs_to :parent
end

I want to create an scope on the child model, that uses the scope of the parent model.

Is there anyhow so that I can use the definition of scope on Parent?

Current solution

The way I'm doing it right now is

class Child < ApplicationRecord
  belongs_to :parent
  scope :wit_great_parent, -> (min_grade) {
        join(:parent).where("grade > :grade", grade: min_grade)}
end

However, I'm copying the where clause in both places.

Question

Is there anyhow to call the Parent scope from the child model?

Answer 1

If you are just looking to merge in the scope then

class Child < ApplicationRecord
  belongs_to :parent
  scope :with_great_parent, -> (min_grade) {joins(:parent).merge(Parent.great(min_grade))}
end

should handle this for you. The SQL generated will be similar to

SELECT * 
FROM children 
  INNER JOIN parents ON children.parent_id = parents.id
WHERE 
  parents.grade > --Whatever value you pass as min_grade

See ActiveRecord::SpawnMethods#merge for more information