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.

10 files changed, 129 insertions, 0 deletions

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,