Add Merge A Scope Into An ActiveRecord Query as a rails til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_818 TILs and counting..._
+_819 TILs and counting..._
 
 ---
 
@@ -504,6 +504,7 @@ _818 TILs and counting..._
 - [Mark A Migration As Irreversible](rails/mark-a-migration-as-irreversible.md)
 - [Make ActionMailer Synchronous In Test](rails/make-action-mailer-synchronous-in-test.md)
 - [Mark For Destruction](rails/mark-for-destruction.md)
+- [Merge A Scope Into An ActiveRecord Query](rails/merge-a-scope-into-an-activerecord-query.md)
 - [Migrating Up Down Up](rails/migrating-up-down-up.md)
 - [Params Includes Submission Button Info](rails/params-includes-submission-button-info.md)
 - [Perform SQL Explain With ActiveRecord](rails/perform-sql-explain-with-activerecord.md)

rails/merge-a-scope-into-an-activerecord-query.md

@@ -0,0 +1,34 @@
+# Merge A Scope Into An ActiveRecord Query
+
+Consider an ActiveRecord model with a scope:
+
+```ruby
+class Book < ApplicationRecord
+  scope :published, -> { where("books.published_at is not null") }
+end
+```
+
+Now let's say we are working in another part of the codebase composing a query
+that gathers all authors with published books. That might look something like
+this:
+
+```ruby
+published_authors =
+  Authors.joins(:book).where("books.published_at is not null")
+```
+
+This will get the job done, but we've now duplicated the same logic in
+different parts of the app. We can utilize the existing scope on `Book` using
+ActiveRecord's
+[`merge`](https://devdocs.io/rails~5.2/activerecord/spawnmethods#method-i-merge)
+method.
+
+```ruby
+published_authors =
+  Authors.joins(:book).merge( Book.published )
+```
+
+The `merge` method can be used to incorporate any conditions from other partial
+queries -- this means both `where` clauses and `joins` clauses.
+
+[source](http://aokolish.me/blog/2015/05/26/how-to-simplify-active-record-scopes-that-reference-other-tables/)