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, 13 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 148b3800a8..adf41cd872 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,16 @@
+*   Add `ActiveRecord::Relation#annotate` for adding SQL comments to its queries.
+
+    For example:
+
+    ```
+    Post.where(id: 123).annotate("this is a comment").to_sql
+    # SELECT "posts".* FROM "posts" WHERE "posts"."id" = 123 /* this is a comment */
+    ```
+
+    This can be useful in instrumentation or other analysis of issued queries.
+
+    *Matt Yoho*
+
 *   Support Optimizer Hints.
 
     In most databases, there is a way to control the optimizer is by using optimizer hints,