association_hook.rb

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t| 
  end

  create_table :comments, force: true do |t| 
    t.integer :post_id
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  class_attribute :default_association_scopes, instance_writer: false, instance_predicate: false, default: []

  def self.default_association_scope(&scope)
    self.default_association_scopes += [scope]
  end

  def self.inherited(klass)
    super

    klass.relation_delegate_class(ActiveRecord::Associations::CollectionProxy).class_eval do
      def self.new(*)
        relation = super

        default_association_scopes = relation.klass.default_association_scopes

        unless default_association_scopes.empty?
          default_association_scopes.inject(relation) do |scope, default_scope|
            scope.merge!(scope.instance_exec(&default_scope))
          end
        end

        relation
      end
    end
  end
end

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post

  default_association_scope {
    p proxy_association.owner # => #<Post id: 1>
    self
  }
end

class AssociationTest < Minitest::Test
  def test_stuff
    post = Post.create!
    post.comments
  end
end