Add Relation#annotate for SQL commenting

This patch has two main portions:

1. Add SQL comment support to Arel via Arel::Nodes::Comment.
2. Implement a Relation#annotate method on top of that.

== Adding SQL comment support

Adds a new Arel::Nodes::Comment node that represents an optional SQL
comment and teachers the relevant visitors how to handle it.

Comment nodes may be added to the basic CRUD statement nodes and set
through any of the four (Select|Insert|Update|Delete)Manager objects.

For example:

    manager = Arel::UpdateManager.new
    manager.table table
    manager.comment("annotation")
    manager.to_sql # UPDATE "users" /* annotation */

This new node type will be used by ActiveRecord::Relation to enable
query annotation via SQL comments.

== Implementing the Relation#annotate method

Implements `ActiveRecord::Relation#annotate`, which accepts a comment
string that will be appeneded to any queries generated by the relation.

Some examples:

    relation = Post.where(id: 123).annotate("metadata string")
    relation.first
    # SELECT "posts".* FROM "posts" WHERE "posts"."id" = 123
    # LIMIT 1 /* metadata string */

    class Tag < ActiveRecord::Base
      scope :foo_annotated, -> { annotate("foo") }
    end
    Tag.foo_annotated.annotate("bar").first
    # SELECT "tags".* FROM "tags" LIMIT 1 /* foo */ /* bar */

Also wires up the plumbing so this works with `#update_all` and
`#delete_all` as well.

This feature is useful for instrumentation and general analysis of
queries generated at runtime.

20 files changed, 497 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/mysql2/annotate_test.rb b/activerecord/test/cases/adapters/mysql2/annotate_test.rb
new file mode 100644
index 0000000000..b512540073
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql2/annotate_test.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/post"
+
+class Mysql2AnnotateTest < ActiveRecord::Mysql2TestCase
+  fixtures :posts
+
+  def test_annotate_wraps_content_in_an_inline_comment
+    assert_sql(%r{\ASELECT `posts`\.`id` FROM `posts` /\* foo \*/}) do
+      posts = Post.select(:id).annotate("foo")
+      assert posts.first
+    end
+  end
+
+  def test_annotate_is_sanitized
+    assert_sql(%r{\ASELECT `posts`\.`id` FROM `posts` /\* foo \*/}) do
+      posts = Post.select(:id).annotate("*/foo/*")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT `posts`\.`id` FROM `posts` /\* foo \*/}) do
+      posts = Post.select(:id).annotate("**//foo//**")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT `posts`\.`id` FROM `posts` /\* foo \*/ /\* bar \*/}) do
+      posts = Post.select(:id).annotate("*/foo/*").annotate("*/bar")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT `posts`\.`id` FROM `posts` /\* \+ MAX_EXECUTION_TIME\(1\) \*/}) do
+      posts = Post.select(:id).annotate("+ MAX_EXECUTION_TIME(1)")
+      assert posts.first
+    end
+  end
+end
diff --git a/activerecord/test/cases/adapters/postgresql/annotate_test.rb b/activerecord/test/cases/adapters/postgresql/annotate_test.rb
new file mode 100644
index 0000000000..42a2861511
--- /dev/null
+++ b/activerecord/test/cases/adapters/postgresql/annotate_test.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/post"
+
+class PostgresqlAnnotateTest < ActiveRecord::PostgreSQLTestCase
+  fixtures :posts
+
+  def test_annotate_wraps_content_in_an_inline_comment
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/}) do
+      posts = Post.select(:id).annotate("foo")
+      assert posts.first
+    end
+  end
+
+  def test_annotate_is_sanitized
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/}) do
+      posts = Post.select(:id).annotate("*/foo/*")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/}) do
+      posts = Post.select(:id).annotate("**//foo//**")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/ /\* bar \*/}) do
+      posts = Post.select(:id).annotate("*/foo/*").annotate("*/bar")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* \+ MAX_EXECUTION_TIME\(1\) \*/}) do
+      posts = Post.select(:id).annotate("+ MAX_EXECUTION_TIME(1)")
+      assert posts.first
+    end
+  end
+end
diff --git a/activerecord/test/cases/adapters/sqlite3/annotate_test.rb b/activerecord/test/cases/adapters/sqlite3/annotate_test.rb
new file mode 100644
index 0000000000..6567a5eca3
--- /dev/null
+++ b/activerecord/test/cases/adapters/sqlite3/annotate_test.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/post"
+
+class SQLite3AnnotateTest < ActiveRecord::SQLite3TestCase
+  fixtures :posts
+
+  def test_annotate_wraps_content_in_an_inline_comment
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/}) do
+      posts = Post.select(:id).annotate("foo")
+      assert posts.first
+    end
+  end
+
+  def test_annotate_is_sanitized
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/}) do
+      posts = Post.select(:id).annotate("*/foo/*")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/}) do
+      posts = Post.select(:id).annotate("**//foo//**")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* foo \*/ /\* bar \*/}) do
+      posts = Post.select(:id).annotate("*/foo/*").annotate("*/bar")
+      assert posts.first
+    end
+
+    assert_sql(%r{\ASELECT "posts"\."id" FROM "posts" /\* \+ MAX_EXECUTION_TIME\(1\) \*/}) do
+      posts = Post.select(:id).annotate("+ MAX_EXECUTION_TIME(1)")
+      assert posts.first
+    end
+  end
+end
diff --git a/activerecord/test/cases/arel/delete_manager_test.rb b/activerecord/test/cases/arel/delete_manager_test.rb
index 0bad02f4d2..63cd1bffe3 100644
--- a/activerecord/test/cases/arel/delete_manager_test.rb
+++ b/activerecord/test/cases/arel/delete_manager_test.rb
@@ -49,5 +49,23 @@ module Arel
         dm.where(table[:id].eq(10)).must_equal dm
       end
     end
+
+    describe "comment" do
+      it "chains" do
+        manager = Arel::DeleteManager.new
+        manager.comment("deleting").must_equal manager
+      end
+
+      it "appends a comment to the generated query" do
+        table = Table.new(:users)
+        dm = Arel::DeleteManager.new
+        dm.from table
+        dm.comment("deletion")
+        assert_match(%r{DELETE FROM "users" /\* deletion \*/}, dm.to_sql)
+
+        dm.comment("deletion", "with", "comment")
+        assert_match(%r{DELETE FROM "users" /\* deletion \*/ /\* with \*/ /\* comment \*/}, dm.to_sql)
+      end
+    end
   end
 end
diff --git a/activerecord/test/cases/arel/nodes/comment_test.rb b/activerecord/test/cases/arel/nodes/comment_test.rb
new file mode 100644
index 0000000000..bf5eaf4c5a
--- /dev/null
+++ b/activerecord/test/cases/arel/nodes/comment_test.rb
@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require_relative "../helper"
+require "yaml"
+
+module Arel
+  module Nodes
+    class CommentTest < Arel::Spec
+      describe "equality" do
+        it "is equal with equal contents" do
+          array = [Comment.new(["foo"]), Comment.new(["foo"])]
+          assert_equal 1, array.uniq.size
+        end
+
+        it "is not equal with different contents" do
+          array = [Comment.new(["foo"]), Comment.new(["bar"])]
+          assert_equal 2, array.uniq.size
+        end
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/arel/nodes/delete_statement_test.rb b/activerecord/test/cases/arel/nodes/delete_statement_test.rb
index 3f078063a4..8ba268653d 100644
--- a/activerecord/test/cases/arel/nodes/delete_statement_test.rb
+++ b/activerecord/test/cases/arel/nodes/delete_statement_test.rb
@@ -18,8 +18,10 @@ describe Arel::Nodes::DeleteStatement do
     it "is equal with equal ivars" do
       statement1 = Arel::Nodes::DeleteStatement.new
       statement1.wheres = %w[a b c]
+      statement1.comment = Arel::Nodes::Comment.new(["comment"])
       statement2 = Arel::Nodes::DeleteStatement.new
       statement2.wheres = %w[a b c]
+      statement2.comment = Arel::Nodes::Comment.new(["comment"])
       array = [statement1, statement2]
       assert_equal 1, array.uniq.size
     end
@@ -27,8 +29,14 @@ describe Arel::Nodes::DeleteStatement do
     it "is not equal with different ivars" do
       statement1 = Arel::Nodes::DeleteStatement.new
       statement1.wheres = %w[a b c]
+      statement1.comment = Arel::Nodes::Comment.new(["comment"])
       statement2 = Arel::Nodes::DeleteStatement.new
       statement2.wheres = %w[1 2 3]
+      statement2.comment = Arel::Nodes::Comment.new(["comment"])
+      array = [statement1, statement2]
+      assert_equal 2, array.uniq.size
+      statement2.wheres = %w[a b c]
+      statement2.comment = Arel::Nodes::Comment.new(["other"])
       array = [statement1, statement2]
       assert_equal 2, array.uniq.size
     end
diff --git a/activerecord/test/cases/arel/nodes/insert_statement_test.rb b/activerecord/test/cases/arel/nodes/insert_statement_test.rb
index 252a0d0d0b..036576b231 100644
--- a/activerecord/test/cases/arel/nodes/insert_statement_test.rb
+++ b/activerecord/test/cases/arel/nodes/insert_statement_test.rb
@@ -23,9 +23,11 @@ describe Arel::Nodes::InsertStatement do
       statement1 = Arel::Nodes::InsertStatement.new
       statement1.columns = %w[a b c]
       statement1.values  = %w[x y z]
+      statement1.comment = Arel::Nodes::Comment.new(["comment"])
       statement2 = Arel::Nodes::InsertStatement.new
       statement2.columns = %w[a b c]
       statement2.values  = %w[x y z]
+      statement2.comment = Arel::Nodes::Comment.new(["comment"])
       array = [statement1, statement2]
       assert_equal 1, array.uniq.size
     end
@@ -34,9 +36,15 @@ describe Arel::Nodes::InsertStatement do
       statement1 = Arel::Nodes::InsertStatement.new
       statement1.columns = %w[a b c]
       statement1.values  = %w[x y z]
+      statement1.comment = Arel::Nodes::Comment.new(["comment"])
       statement2 = Arel::Nodes::InsertStatement.new
       statement2.columns = %w[a b c]
       statement2.values  = %w[1 2 3]
+      statement2.comment = Arel::Nodes::Comment.new(["comment"])
+      array = [statement1, statement2]
+      assert_equal 2, array.uniq.size
+      statement2.values  = %w[x y z]
+      statement2.comment = Arel::Nodes::Comment.new("other")
       array = [statement1, statement2]
       assert_equal 2, array.uniq.size
     end
diff --git a/activerecord/test/cases/arel/nodes/select_core_test.rb b/activerecord/test/cases/arel/nodes/select_core_test.rb
index 0b698205ff..6860f2a395 100644
--- a/activerecord/test/cases/arel/nodes/select_core_test.rb
+++ b/activerecord/test/cases/arel/nodes/select_core_test.rb
@@ -37,6 +37,7 @@ module Arel
         core1.groups      = %w[j k l]
         core1.windows     = %w[m n o]
         core1.havings     = %w[p q r]
+        core1.comment     = Arel::Nodes::Comment.new(["comment"])
         core2 = SelectCore.new
         core2.froms       = %w[a b c]
         core2.projections = %w[d e f]
@@ -44,6 +45,7 @@ module Arel
         core2.groups      = %w[j k l]
         core2.windows     = %w[m n o]
         core2.havings     = %w[p q r]
+        core2.comment     = Arel::Nodes::Comment.new(["comment"])
         array = [core1, core2]
         assert_equal 1, array.uniq.size
       end
@@ -56,6 +58,7 @@ module Arel
         core1.groups      = %w[j k l]
         core1.windows     = %w[m n o]
         core1.havings     = %w[p q r]
+        core1.comment     = Arel::Nodes::Comment.new(["comment"])
         core2 = SelectCore.new
         core2.froms       = %w[a b c]
         core2.projections = %w[d e f]
@@ -63,6 +66,11 @@ module Arel
         core2.groups      = %w[j k l]
         core2.windows     = %w[m n o]
         core2.havings     = %w[l o l]
+        core2.comment     = Arel::Nodes::Comment.new(["comment"])
+        array = [core1, core2]
+        assert_equal 2, array.uniq.size
+        core2.havings     = %w[p q r]
+        core2.comment     = Arel::Nodes::Comment.new(["other"])
         array = [core1, core2]
         assert_equal 2, array.uniq.size
       end
diff --git a/activerecord/test/cases/arel/nodes/update_statement_test.rb b/activerecord/test/cases/arel/nodes/update_statement_test.rb
index a83ce32f68..f133ddf7eb 100644
--- a/activerecord/test/cases/arel/nodes/update_statement_test.rb
+++ b/activerecord/test/cases/arel/nodes/update_statement_test.rb
@@ -27,6 +27,7 @@ describe Arel::Nodes::UpdateStatement do
       statement1.orders   = %w[x y z]
       statement1.limit    = 42
       statement1.key      = "zomg"
+      statement1.comment = Arel::Nodes::Comment.new(["comment"])
       statement2 = Arel::Nodes::UpdateStatement.new
       statement2.relation = "zomg"
       statement2.wheres   = 2
@@ -34,6 +35,7 @@ describe Arel::Nodes::UpdateStatement do
       statement2.orders   = %w[x y z]
       statement2.limit    = 42
       statement2.key      = "zomg"
+      statement2.comment = Arel::Nodes::Comment.new(["comment"])
       array = [statement1, statement2]
       assert_equal 1, array.uniq.size
     end
@@ -46,6 +48,7 @@ describe Arel::Nodes::UpdateStatement do
       statement1.orders   = %w[x y z]
       statement1.limit    = 42
       statement1.key      = "zomg"
+      statement1.comment = Arel::Nodes::Comment.new(["comment"])
       statement2 = Arel::Nodes::UpdateStatement.new
       statement2.relation = "zomg"
       statement2.wheres   = 2
@@ -53,6 +56,11 @@ describe Arel::Nodes::UpdateStatement do
       statement2.orders   = %w[x y z]
       statement2.limit    = 42
       statement2.key      = "wth"
+      statement2.comment = Arel::Nodes::Comment.new(["comment"])
+      array = [statement1, statement2]
+      assert_equal 2, array.uniq.size
+      statement2.key      = "zomg"
+      statement2.comment  = Arel::Nodes::Comment.new(["other"])
       array = [statement1, statement2]
       assert_equal 2, array.uniq.size
     end
diff --git a/activerecord/test/cases/arel/select_manager_test.rb b/activerecord/test/cases/arel/select_manager_test.rb
index 5220950905..e6c49cd429 100644
--- a/activerecord/test/cases/arel/select_manager_test.rb
+++ b/activerecord/test/cases/arel/select_manager_test.rb
@@ -1221,5 +1221,28 @@ module Arel
         manager.distinct_on(false).must_equal manager
       end
     end
+
+    describe "comment" do
+      it "chains" do
+        manager = Arel::SelectManager.new
+        manager.comment("selecting").must_equal manager
+      end
+
+      it "appends a comment to the generated query" do
+        manager = Arel::SelectManager.new
+        table = Table.new :users
+        manager.from(table).project(table["id"])
+
+        manager.comment("selecting")
+        manager.to_sql.must_be_like %{
+          SELECT "users"."id" FROM "users" /* selecting */
+        }
+
+        manager.comment("selecting", "with", "comment")
+        manager.to_sql.must_be_like %{
+          SELECT "users"."id" FROM "users" /* selecting */ /* with */ /* comment */
+        }
+      end
+    end
   end
 end
diff --git a/activerecord/test/cases/arel/support/fake_record.rb b/activerecord/test/cases/arel/support/fake_record.rb
index 559ff5d4e6..18e6c10c9d 100644
--- a/activerecord/test/cases/arel/support/fake_record.rb
+++ b/activerecord/test/cases/arel/support/fake_record.rb
@@ -58,6 +58,10 @@ module FakeRecord
       "\"#{name}\""
     end
 
+    def sanitize_as_sql_comment(comment)
+      comment
+    end
+
     def schema_cache
       self
     end
diff --git a/activerecord/test/cases/arel/update_manager_test.rb b/activerecord/test/cases/arel/update_manager_test.rb
index cc1b9ac5b3..e13cb6aa52 100644
--- a/activerecord/test/cases/arel/update_manager_test.rb
+++ b/activerecord/test/cases/arel/update_manager_test.rb
@@ -122,5 +122,29 @@ module Arel
         @um.key.must_equal @table[:foo]
       end
     end
+
+    describe "comment" do
+      it "chains" do
+        manager = Arel::UpdateManager.new
+        manager.comment("updating").must_equal manager
+      end
+
+      it "appends a comment to the generated query" do
+        table   = Table.new :users
+
+        manager = Arel::UpdateManager.new
+        manager.table table
+
+        manager.comment("updating")
+        manager.to_sql.must_be_like %{
+          UPDATE "users" /* updating */
+        }
+
+        manager.comment("updating", "with", "comment")
+        manager.to_sql.must_be_like %{
+          UPDATE "users" /* updating */ /* with */ /* comment */
+        }
+      end
+    end
   end
 end
diff --git a/activerecord/test/cases/arel/visitors/depth_first_test.rb b/activerecord/test/cases/arel/visitors/depth_first_test.rb
index 4a57608411..106be2311d 100644
--- a/activerecord/test/cases/arel/visitors/depth_first_test.rb
+++ b/activerecord/test/cases/arel/visitors/depth_first_test.rb
@@ -101,6 +101,12 @@ module Arel
         assert_equal [:a, :b, join], @collector.calls
       end
 
+      def test_comment
+        comment = Nodes::Comment.new ["foo"]
+        @visitor.accept comment
+        assert_equal ["foo", ["foo"], comment], @collector.calls
+      end
+
       [
         Arel::Nodes::Assignment,
         Arel::Nodes::Between,
diff --git a/activerecord/test/cases/associations_test.rb b/activerecord/test/cases/associations_test.rb
index 081da95df7..84130ec208 100644
--- a/activerecord/test/cases/associations_test.rb
+++ b/activerecord/test/cases/associations_test.rb
@@ -21,6 +21,11 @@ require "models/molecule"
 require "models/electron"
 require "models/man"
 require "models/interest"
+require "models/pirate"
+require "models/parrot"
+require "models/bird"
+require "models/treasure"
+require "models/price_estimate"
 
 class AssociationsTest < ActiveRecord::TestCase
   fixtures :accounts, :companies, :developers, :projects, :developers_projects,
@@ -368,3 +373,97 @@ class GeneratedMethodsTest < ActiveRecord::TestCase
     assert_equal :none, MyArticle.new.comments
   end
 end
+
+class WithAnnotationsTest < ActiveRecord::TestCase
+  fixtures :pirates, :parrots
+
+  def test_belongs_to_with_annotation_includes_a_query_comment
+    pirate = SpacePirate.where.not(parrot_id: nil).first
+    assert pirate, "should have a Pirate record"
+
+    log = capture_sql do
+      pirate.parrot
+    end
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    assert_sql(%r{/\* that tells jokes \*/}) do
+      pirate.parrot_with_annotation
+    end
+  end
+
+  def test_has_and_belongs_to_many_with_annotation_includes_a_query_comment
+    pirate = SpacePirate.first
+    assert pirate, "should have a Pirate record"
+
+    log = capture_sql do
+      pirate.parrots.first
+    end
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    assert_sql(%r{/\* that are very colorful \*/}) do
+      pirate.parrots_with_annotation.first
+    end
+  end
+
+  def test_has_one_with_annotation_includes_a_query_comment
+    pirate = SpacePirate.first
+    assert pirate, "should have a Pirate record"
+
+    log = capture_sql do
+      pirate.ship
+    end
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    assert_sql(%r{/\* that is a rocket \*/}) do
+      pirate.ship_with_annotation
+    end
+  end
+
+  def test_has_many_with_annotation_includes_a_query_comment
+    pirate = SpacePirate.first
+    assert pirate, "should have a Pirate record"
+
+    log = capture_sql do
+      pirate.birds.first
+    end
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    assert_sql(%r{/\* that are also parrots \*/}) do
+      pirate.birds_with_annotation.first
+    end
+  end
+
+  def test_has_many_through_with_annotation_includes_a_query_comment
+    pirate = SpacePirate.first
+    assert pirate, "should have a Pirate record"
+
+    log = capture_sql do
+      pirate.treasure_estimates.first
+    end
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    assert_sql(%r{/\* yarrr \*/}) do
+      pirate.treasure_estimates_with_annotation.first
+    end
+  end
+
+  def test_has_many_through_with_annotation_includes_a_query_comment_when_eager_loading
+    pirate = SpacePirate.first
+    assert pirate, "should have a Pirate record"
+
+    log = capture_sql do
+      pirate.treasure_estimates.first
+    end
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    assert_sql(%r{/\* yarrr \*/}) do
+      SpacePirate.includes(:treasure_estimates_with_annotation, :treasures).first
+    end
+  end
+end
diff --git a/activerecord/test/cases/relation/delete_all_test.rb b/activerecord/test/cases/relation/delete_all_test.rb
index d1c13fa1b5..9b76936b7e 100644
--- a/activerecord/test/cases/relation/delete_all_test.rb
+++ b/activerecord/test/cases/relation/delete_all_test.rb
@@ -99,4 +99,23 @@ class DeleteAllTest < ActiveRecord::TestCase
     assert_raise(ActiveRecord::RecordNotFound) { posts(:thinking) }
     assert posts(:welcome)
   end
+
+  def test_delete_all_with_annotation_includes_a_query_comment
+    davids = Author.where(name: "David").annotate("deleting all")
+
+    assert_sql(%r{/\* deleting all \*/}) do
+      assert_difference("Author.count", -1) { davids.delete_all }
+    end
+  end
+
+  def test_delete_all_without_annotation_does_not_include_an_empty_comment
+    davids = Author.where(name: "David")
+
+    log = capture_sql do
+      assert_difference("Author.count", -1) { davids.delete_all }
+    end
+
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+  end
 end
diff --git a/activerecord/test/cases/relation/merging_test.rb b/activerecord/test/cases/relation/merging_test.rb
index 224e4f39a8..5c5e760e34 100644
--- a/activerecord/test/cases/relation/merging_test.rb
+++ b/activerecord/test/cases/relation/merging_test.rb
@@ -135,6 +135,18 @@ class RelationMergingTest < ActiveRecord::TestCase
     relation = Post.all.merge(Post.order(Arel.sql("title LIKE '%?'")))
     assert_equal ["title LIKE '%?'"], relation.order_values
   end
+
+  def test_merging_annotations_respects_merge_order
+    assert_sql(%r{/\* foo \*/ /\* bar \*/}) do
+      Post.annotate("foo").merge(Post.annotate("bar")).first
+    end
+    assert_sql(%r{/\* bar \*/ /\* foo \*/}) do
+      Post.annotate("bar").merge(Post.annotate("foo")).first
+    end
+    assert_sql(%r{/\* foo \*/ /\* bar \*/ /\* baz \*/ /\* qux \*/}) do
+      Post.annotate("foo").annotate("bar").merge(Post.annotate("baz").annotate("qux")).first
+    end
+  end
 end
 
 class MergingDifferentRelationsTest < ActiveRecord::TestCase
diff --git a/activerecord/test/cases/relation/update_all_test.rb b/activerecord/test/cases/relation/update_all_test.rb
index 0500574f28..526f926841 100644
--- a/activerecord/test/cases/relation/update_all_test.rb
+++ b/activerecord/test/cases/relation/update_all_test.rb
@@ -241,6 +241,33 @@ class UpdateAllTest < ActiveRecord::TestCase
     end
   end
 
+  def test_update_all_with_annotation_includes_a_query_comment
+    tag = Tag.first
+
+    assert_sql(%r{/\* updating all \*/}) do
+      Post.tagged_with(tag.id).annotate("updating all").update_all(title: "rofl")
+    end
+
+    posts = Post.tagged_with(tag.id).all.to_a
+    assert_operator posts.length, :>, 0
+    posts.each { |post| assert_equal "rofl", post.title }
+  end
+
+  def test_update_all_without_annotation_does_not_include_an_empty_comment
+    tag = Tag.first
+
+    log = capture_sql do
+      Post.tagged_with(tag.id).update_all(title: "rofl")
+    end
+
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+
+    posts = Post.tagged_with(tag.id).all.to_a
+    assert_operator posts.length, :>, 0
+    posts.each { |post| assert_equal "rofl", post.title }
+  end
+
   # Oracle UPDATE does not support ORDER BY
   unless current_adapter?(:OracleAdapter)
     def test_update_all_ignores_order_without_limit_from_association
diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index 68161f6a84..00a7b3841f 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -307,6 +307,58 @@ module ActiveRecord
       assert_equal 3, ratings.count
     end
 
+    def test_relation_with_annotation_includes_comment_in_to_sql
+      post_with_annotation = Post.where(id: 1).annotate("foo")
+      assert_match %r{= 1 /\* foo \*/}, post_with_annotation.to_sql
+    end
+
+    def test_relation_with_annotation_includes_comment_in_sql
+      post_with_annotation = Post.where(id: 1).annotate("foo")
+      assert_sql(%r{/\* foo \*/}) do
+        assert post_with_annotation.first, "record should be found"
+      end
+    end
+
+    def test_relation_with_annotation_chains_sql_comments
+      post_with_annotation = Post.where(id: 1).annotate("foo").annotate("bar")
+      assert_sql(%r{/\* foo \*/ /\* bar \*/}) do
+        assert post_with_annotation.first, "record should be found"
+      end
+    end
+
+    def test_relation_with_annotation_filters_sql_comment_delimiters
+      post_with_annotation = Post.where(id: 1).annotate("**//foo//**")
+      assert_match %r{= 1 /\* foo \*/}, post_with_annotation.to_sql
+    end
+
+    def test_relation_with_annotation_includes_comment_in_count_query
+      post_with_annotation = Post.annotate("foo")
+      all_count = Post.all.to_a.count
+      assert_sql(%r{/\* foo \*/}) do
+        assert_equal all_count, post_with_annotation.count
+      end
+    end
+
+    def test_relation_without_annotation_does_not_include_an_empty_comment
+      log = capture_sql do
+        Post.where(id: 1).first
+      end
+
+      assert_not_predicate log, :empty?
+      assert_predicate log.select { |query| query.match?(%r{/\*}) }, :empty?
+    end
+
+    def test_relation_with_optimizer_hints_filters_sql_comment_delimiters
+      post_with_hint = Post.where(id: 1).optimizer_hints("**//BADHINT//**")
+      assert_match %r{BADHINT}, post_with_hint.to_sql
+      assert_no_match %r{\*/BADHINT}, post_with_hint.to_sql
+      assert_no_match %r{\*//BADHINT}, post_with_hint.to_sql
+      assert_no_match %r{BADHINT/\*}, post_with_hint.to_sql
+      assert_no_match %r{BADHINT//\*}, post_with_hint.to_sql
+      post_with_hint = Post.where(id: 1).optimizer_hints("/*+ BADHINT */")
+      assert_match %r{/\*\+ BADHINT \*/}, post_with_hint.to_sql
+    end
+
     class EnsureRoundTripTypeCasting < ActiveRecord::Type::Value
       def type
         :string
diff --git a/activerecord/test/cases/scoping/named_scoping_test.rb b/activerecord/test/cases/scoping/named_scoping_test.rb
index 8b08e40468..3488442cab 100644
--- a/activerecord/test/cases/scoping/named_scoping_test.rb
+++ b/activerecord/test/cases/scoping/named_scoping_test.rb
@@ -602,4 +602,14 @@ class NamedScopingTest < ActiveRecord::TestCase
     Topic.create!
     assert_predicate Topic, :one?
   end
+
+  def test_scope_with_annotation
+    Topic.class_eval do
+      scope :including_annotate_in_scope, Proc.new { annotate("from-scope") }
+    end
+
+    assert_sql(%r{/\* from-scope \*/}) do
+      assert Topic.including_annotate_in_scope.to_a, Topic.all.to_a
+    end
+  end
 end
diff --git a/activerecord/test/cases/scoping/relation_scoping_test.rb b/activerecord/test/cases/scoping/relation_scoping_test.rb
index b1f2ffe29c..a95ab0f429 100644
--- a/activerecord/test/cases/scoping/relation_scoping_test.rb
+++ b/activerecord/test/cases/scoping/relation_scoping_test.rb
@@ -130,6 +130,44 @@ class RelationScopingTest < ActiveRecord::TestCase
     end
   end
 
+  def test_scoped_find_with_annotation
+    Developer.annotate("scoped").scoping do
+      developer = nil
+      assert_sql(%r{/\* scoped \*/}) do
+        developer = Developer.where("name = 'David'").first
+      end
+      assert_equal "David", developer.name
+    end
+  end
+
+  def test_find_with_annotation_unscoped
+    Developer.annotate("scoped").unscoped do
+      developer = nil
+      log = capture_sql do
+        developer = Developer.where("name = 'David'").first
+      end
+
+      assert_not_predicate log, :empty?
+      assert_predicate log.select { |query| query.match?(%r{/\* scoped \*/}) }, :empty?
+
+      assert_equal "David", developer.name
+    end
+  end
+
+  def test_find_with_annotation_unscope
+    developer = nil
+    log = capture_sql do
+      developer = Developer.annotate("unscope").
+        where("name = 'David'").
+        unscope(:annotate).first
+    end
+
+    assert_not_predicate log, :empty?
+    assert_predicate log.select { |query| query.match?(%r{/\* unscope \*/}) }, :empty?
+
+    assert_equal "David", developer.name
+  end
+
   def test_scoped_find_include
     # with the include, will retrieve only developers for the given project
     scoped_developers = Developer.includes(:projects).scoping do