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, 9 insertions, 3 deletions

diff --git a/activerecord/lib/arel/visitors/to_sql.rb b/activerecord/lib/arel/visitors/to_sql.rb
index 1630226085..4192d9efdc 100644
--- a/activerecord/lib/arel/visitors/to_sql.rb
+++ b/activerecord/lib/arel/visitors/to_sql.rb
@@ -35,6 +35,7 @@ module Arel # :nodoc: all
           collect_nodes_for o.wheres, collector, " WHERE ", " AND "
           collect_nodes_for o.orders, collector, " ORDER BY "
           maybe_visit o.limit, collector
+          maybe_visit o.comment, collector
         end
 
         def visit_Arel_Nodes_UpdateStatement(o, collector)
@@ -47,6 +48,7 @@ module Arel # :nodoc: all
           collect_nodes_for o.wheres, collector, " WHERE ", " AND "
           collect_nodes_for o.orders, collector, " ORDER BY "
           maybe_visit o.limit, collector
+          maybe_visit o.comment, collector
         end
 
         def visit_Arel_Nodes_InsertStatement(o, collector)
@@ -62,9 +64,9 @@ module Arel # :nodoc: all
             maybe_visit o.values, collector
           elsif o.select
             maybe_visit o.select, collector
-          else
-            collector
           end
+
+          maybe_visit o.comment, collector
         end
 
         def visit_Arel_Nodes_Exists(o, collector)
@@ -162,7 +164,7 @@ module Arel # :nodoc: all
           collect_nodes_for o.havings, collector, " HAVING ", " AND "
           collect_nodes_for o.windows, collector, " WINDOW "
 
-          collector
+          maybe_visit o.comment, collector
         end
 
         def visit_Arel_Nodes_OptimizerHints(o, collector)
@@ -170,6 +172,10 @@ module Arel # :nodoc: all
           collector << "/*+ #{hints} */"
         end
 
+        def visit_Arel_Nodes_Comment(o, collector)
+          collector << o.values.map { |v| "/* #{sanitize_as_sql_comment(v)} */" }.join(" ")
+        end
+
         def collect_nodes_for(nodes, collector, spacer, connector = ", ")
           unless nodes.empty?
             collector << spacer