Apply scope to association subqueries

[lazaronixon]

Motivation / Background

Apply scope to association subqueries. (belongs_to/has_one/has_many).

Given:

class Author < ApplicationRecord
  has_many :welcome_posts, -> { where(title: "welcome") } , class_name: "Post"
end

Before:

Author.where(welcome_posts: Post.all)
#=> SELECT (...) WHERE "authors"."id" IN (SELECT "posts"."author_id" FROM "posts")

Later:

Author.where(welcome_posts: Post.all)
#=> SELECT (...) WHERE "authors"."id" IN (SELECT "posts"."author_id" FROM "posts" WHERE "posts"."title" = 'welcome')

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[rafaelfranca]

This is a huge breaking change. Conditions that were not applied in previous versions of Rails will be applied now giving people different results.

@lazaronixon can you open a PR to make this behavior opt-in when an option is passed to the association? Like has_many :welcome_posts, -> { where(title: "welcome") } , class_name: "Post", apply_on_subqueries: true

[aharpole]

Another thing this change breaks is that if you pass a block that takes a parameter in as a scope, trying to use the association in a subquery now results in an ArgumentError.

luanzeba#4 provides a failing test.

Making this behavior opt in provides us a workaround in the meantime.