From ccdf6b8d42b2d32e0618870c87b1de83c06542c7 Mon Sep 17 00:00:00 2001
From: Kasper Timm Hansen <kaspth@gmail.com>
Date: Thu, 26 Nov 2020 14:15:18 +0100
Subject: [PATCH] Add where.associated to check association presence

---
 activerecord/CHANGELOG.md                     | 14 ++++++++++
 .../active_record/relation/query_methods.rb   | 28 +++++++++++++++++++
 .../test/cases/relation/where_chain_test.rb   | 16 +++++++++++
 3 files changed, 58 insertions(+)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index e2adc7ba98..0ecbcdefc5 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,17 @@
+*   Add `where.associated` to check for the presence of an association.
+
+    ```ruby
+    # Before:
+    account.users.joins(:contact).where.not(contact_id: nil)
+
+    # After:
+    account.users.where.associated(:contact)
+    ```
+
+    Also mirrors `where.missing`.
+
+    *Kasper Timm Hansen*
+
 *   Fix odd behavior of inverse_of with multiple belongs_to to same class.
 
     Fixes #35204.
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index fbf2865811..87cc1af703 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -48,6 +48,34 @@ module ActiveRecord
         @scope
       end
 
+      # Returns a new relation with joins and where clause to identify
+      # associated relations.
+      #
+      # For example, posts that are associated to a related author:
+      #
+      #    Post.where.associated(:author)
+      #    # SELECT "posts".* FROM "posts"
+      #    # INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
+      #    # WHERE "authors"."id" IS NOT NULL
+      #
+      # Additionally, multiple relations can be combined. This will return posts
+      # associated to both an author and any comments:
+      #
+      #    Post.where.associated(:author, :comments)
+      #    # SELECT "posts".* FROM "posts"
+      #    # INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
+      #    # INNER JOIN "comments" ON "comments"."post_id" = "posts"."id"
+      #    # WHERE "authors"."id" IS NOT NULL AND "comments"."id" IS NOT NULL
+      def associated(*associations)
+        associations.each do |association|
+          reflection = @scope.klass._reflect_on_association(association)
+          @scope.joins!(association)
+          self.not(reflection.table_name => { reflection.association_primary_key => nil })
+        end
+
+        @scope
+      end
+
       # Returns a new relation with left outer joins and where clause to identify
       # missing relations.
       #
diff --git a/activerecord/test/cases/relation/where_chain_test.rb b/activerecord/test/cases/relation/where_chain_test.rb
index 8cec633b39..23599ba4cc 100644
--- a/activerecord/test/cases/relation/where_chain_test.rb
+++ b/activerecord/test/cases/relation/where_chain_test.rb
@@ -12,6 +12,22 @@ module ActiveRecord
   class WhereChainTest < ActiveRecord::TestCase
     fixtures :posts, :comments, :authors, :humans, :essays
 
+    def test_associated_with_association
+      Post.where.associated(:author).tap do |relation|
+        assert_includes     relation, posts(:welcome)
+        assert_includes     relation, posts(:sti_habtm)
+        assert_not_includes relation, posts(:authorless)
+      end
+    end
+
+    def test_associated_with_multiple_associations
+      Post.where.associated(:author, :comments).tap do |relation|
+        assert_includes     relation, posts(:welcome)
+        assert_not_includes relation, posts(:sti_habtm)
+        assert_not_includes relation, posts(:authorless)
+      end
+    end
+
     def test_missing_with_association
       assert posts(:authorless).author.blank?
       assert_equal [posts(:authorless)], Post.where.missing(:author).to_a