Introducing the `where.missing` query method.

activerecord/CHANGELOG.md

+*   Find orphans by looking for missing relations through chaining `where.missing`:
+
+    Before:
+
+    ```ruby
+    Post.left_joins(:author).where(authors: { id: nil })
+    ```
+
+    After:
+
+    ```ruby
+    Post.where.missing(:author)
+    ```
+
+    *Tom Rossi*
+
 *   Ensure `:reading` connections always raise if a write is attempted.
 
     Now Rails will raise an `ActiveRecord::ReadOnlyError` if any connection on the reading handler attempts to make a write. If your reading role needs to write you should name the role something other than `:reading`.

activerecord/lib/active_record/relation/query_methods.rb

@@ -70,6 +70,35 @@ def not(opts, *rest)
         @scope
       end
 
+      # Returns a new relation with left outer joins and where clause to idenitfy
+      # missing relations.
+      #
+      # For example, posts that are missing a related author:
+      #
+      #    Post.where.missing(:author)
+      #    SELECT "posts".* FROM "posts"
+      #        LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
+      #        WHERE "authors"."id" IS NULL
+      #
+      #  Additionally, multiple relations can be combined. This will retrun posts
+      #  that are missing both an author and any comments:
+      #
+      #    Post.where.missing(:author, :comments)
+      #    SELECT "posts".* FROM "posts"
+      #        LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
+      #        LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id"
+      #        WHERE "authors"."id" IS NULL AND "comments"."id" IS NULL
+      def missing(*args)
+        args.each do |arg|
+          reflection = @scope.klass._reflect_on_association(arg)
+          opts = { reflection.table_name => { reflection.association_primary_key => nil } }
+          @scope.left_outer_joins!(arg)
+          @scope.where!(opts)
+        end
+
+        @scope
+      end
+
       private
         def not_behaves_as_nor?(opts)
           return false unless opts.is_a?(Hash)

activerecord/test/cases/relation/where_chain_test.rb

@@ -2,17 +2,30 @@
 
 require "cases/helper"
 require "models/post"
+require "models/author"
 require "models/comment"
+require "models/categorization"
+
 
 module ActiveRecord
   class WhereChainTest < ActiveRecord::TestCase
-    fixtures :posts
+    fixtures :posts, :authors
 
     def setup
       super
       @name = "title"
     end
 
+    def test_missing_with_association
+      assert posts(:authorless).author.blank?
+      assert_equal [posts(:authorless)], Post.where.missing(:author).to_a
+    end
+
+    def test_missing_with_multiple_association
+      assert posts(:authorless).comments.empty?
+      assert_equal [posts(:authorless)], Post.where.missing(:author, :comments).to_a
+    end
+
     def test_not_inverts_where_clause
       relation = Post.where.not(title: "hello")
       expected_where_clause = Post.where(title: "hello").where_clause.invert