How can I use a scope in a join query? - ruby-on-rails

Question

I want to use a scope of a joined table.
The goal is to write a scope for autors that have reports with a specific stat_id (for example 15)
Rails 5.2.3
class Author < ApplicationRecord
belongs_to :report
class Report < ApplicationRecord
has_many :authors
scope :with_stat, ->(s) {
where(stat_id: s)
}
This works fine:
Autor.joins(:report).where(reports: {stat_id: 15})
If the scope is more complex. How can I use the scope from class Report?
This doesn't work:
Autor.joins(:report).where(reports: {with_stat(15)})
What is the correct syntax?

Answer 1

That scope will not give you the correct query.
What you want is Author.joins(:report).where(reports: { stat_id: 1 }). Which gives a single query:
Author Load (1.0ms) SELECT "authors".* FROM "authors" INNER JOIN "reports" ON "reports"."id" = "authors"."report_id" WHERE "reports"."stat_id" = $1 LIMIT $2
This is what happens if you use the scope instead:
irb(main):004:0> Author.joins(:report).where(Report.with_stat(1))
Report Load (1.6ms) SELECT "reports".* FROM "reports" WHERE "reports"."stat_id" = $1 [["stat_id", 1]]
Author Load (0.6ms) SELECT "authors".* FROM "authors" INNER JOIN "reports" ON "reports"."id" = "authors"."report_id" LIMIT $1 [["LIMIT", 11]]
=> #<ActiveRecord::Relation []>
irb(main):005:0> Author.joins(:report).where(report: Report.with_stat(1))
Author Load (2.1ms) SELECT "authors".* FROM "authors" INNER JOIN "reports" ON "reports"."id" = "authors"."report_id" WHERE "authors"."report_id" IN (SELECT "reports"."id" FROM "reports" WHERE "reports"."stat_id" = $1) LIMIT $2 [["stat_id", 1], ["LIMIT", 11]]
=> #<ActiveRecord::Relation []>
The later uses a subquery which should give the same result but should be less effective.
What you can do is place the scope on the other side of the association:
class Author < ApplicationRecord
belongs_to :report
scope :with_stat, ->(s){
joins(:report).where(reports: {stat_id: s})
}
end
irb(main):010:0> Author.joins(:report).where(reports: { stat_id: 1 })
Author Load (1.1ms) SELECT "authors".* FROM "authors" INNER JOIN "reports" ON "reports"."id" = "authors"."report_id" WHERE "reports"."stat_id" = $1 LIMIT $2 [["stat_id", 1], ["LIMIT", 11]]
=> #<ActiveRecord::Relation []>