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?

Alacrity

the child has only one parent, so i don't think it makes much sense to scope a single object :/ perhaps return nil if parent.grade > min_grade ? – Blandish

I want to return all the Children that have a parent with greater grade. I don't want to get all of the children and check one by one by the "if" that you propose. – Alacrity

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

Endoparasite

Yes! thats it! Thanks – Alacrity

Answer 2

Hmm, perhaps, you just need to put parents. before grade > :grade query in Child's scope. And you misspelled joins method.

Try use scope on Child model like so:

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

Lakitalaks