Using Named Scopes Across Models with ActiveRecord#Merge

After years of working with ActiveRecord and watching it change so much, it is exciting to find new features you didn't know about. The one I discovered this week is ActiveRecord#merge. It is one of the most underused methods in ActiveRecord, due in part, to the name. It isn't necessarily clear what they mean by "merge" but it's simply a way of using a named scope on a joined model.

Say we have two models that are associated and one of them has a scope:

class Author < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :author

  scope :available, ->{ where(available: true) }
end

Let's say we want to join the tables to find all Authors who have books that are available. Without ActiveRecord#merge, we have to make this query:

Author.joins(:books).where("books.available = ?", true)

SELECT "authors".* FROM "authors" INNER JOIN "books" ON "books"."author_id" = "authors"."id" WHERE "books"."available" = 't'

But with ActiveRecord#merge, this becomes a whole lot cleaner and we don't duplicate the available scope:

Author.joins(:books).merge(Book.available)

SELECT "authors".* FROM "authors" INNER JOIN "books" ON "books"."author_id" = "authors"."id" WHERE "books"."available" = 't'

As you can see, the resulting SQL queries are exactly the same. ActiveRecord#merge is a great way to reduce the duplication in your code to continue relying on the named scopes you define in your models. I really want to see more people using this so please share this around!

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