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.

1 files changed, 99 insertions, 0 deletions

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