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.

15 files changed, 118 insertions, 19 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 32653956b2..db89b77629 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -61,11 +61,15 @@ module ActiveRecord
             scope = through_reflection.klass.unscoped
             options = reflection.options
 
+            values = reflection_scope.values
+            if annotations = values[:annotate]
+              scope.annotate!(*annotations)
+            end
+
             if options[:source_type]
               scope.where! reflection.foreign_type => options[:source_type]
             elsif !reflection_scope.where_clause.empty?
               scope.where_clause = reflection_scope.where_clause
-              values = reflection_scope.values
 
               if includes = values[:includes]
                 scope.includes!(source_reflection.name => includes)
diff --git a/activerecord/lib/active_record/querying.rb b/activerecord/lib/active_record/querying.rb
index 86021f0a80..81ab502824 100644
--- a/activerecord/lib/active_record/querying.rb
+++ b/activerecord/lib/active_record/querying.rb
@@ -15,7 +15,7 @@ module ActiveRecord
       :select, :reselect, :order, :reorder, :group, :limit, :offset, :joins, :left_joins, :left_outer_joins,
       :where, :rewhere, :preload, :eager_load, :includes, :from, :lock, :readonly, :extending, :or,
       :having, :create_with, :distinct, :references, :none, :unscope, :optimizer_hints, :merge, :except, :only,
-      :count, :average, :minimum, :maximum, :sum, :calculate,
+      :count, :average, :minimum, :maximum, :sum, :calculate, :annotate,
       :pluck, :pick, :ids
     ].freeze # :nodoc:
     delegate(*QUERYING_METHODS, to: :all)
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 6d954a2b2e..36c2422d84 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -5,7 +5,7 @@ module ActiveRecord
   class Relation
     MULTI_VALUE_METHODS  = [:includes, :eager_load, :preload, :select, :group,
                             :order, :joins, :left_outer_joins, :references,
-                            :extending, :unscope, :optimizer_hints]
+                            :extending, :unscope, :optimizer_hints, :annotate]
 
     SINGLE_VALUE_METHODS = [:limit, :offset, :lock, :readonly, :reordering,
                             :reverse_order, :distinct, :create_with, :skip_query_cache]
@@ -389,6 +389,8 @@ module ActiveRecord
         stmt.set Arel.sql(klass.sanitize_sql_for_assignment(updates, table.name))
       end
 
+      stmt.comment(*arel.comment_node.values) if arel.comment_node
+
       @klass.connection.update stmt, "#{@klass} Update All"
     end
 
@@ -504,6 +506,7 @@ module ActiveRecord
       stmt.offset(arel.offset)
       stmt.order(*arel.orders)
       stmt.wheres = arel.constraints
+      stmt.comment(*arel.comment_node.values) if arel.comment_node
 
       affected = @klass.connection.delete(stmt, "#{@klass} Destroy")
 
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 05ea0850d3..6f0f2125dc 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -349,7 +349,7 @@ module ActiveRecord
     end
 
     VALID_UNSCOPING_VALUES = Set.new([:where, :select, :group, :order, :lock,
-                                     :limit, :offset, :joins, :left_outer_joins,
+                                     :limit, :offset, :joins, :left_outer_joins, :annotate,
                                      :includes, :from, :readonly, :having, :optimizer_hints])
 
     # Removes an unwanted relation that is already defined on a chain of relations.
@@ -948,6 +948,26 @@ module ActiveRecord
       self
     end
 
+    # Adds an SQL comment to queries generated from this relation. For example:
+    #
+    #   User.annotate("selecting user names").select(:name)
+    #   # SELECT "users"."name" FROM "users" /* selecting user names */
+    #
+    #   User.annotate("selecting", "user", "names").select(:name)
+    #   # SELECT "users"."name" FROM "users" /* selecting */ /* user */ /* names */
+    #
+    # The SQL block comment delimiters, "/*" and "*/", will be added automatically.
+    def annotate(*args)
+      check_if_method_has_arguments!(:annotate, args)
+      spawn.annotate!(*args)
+    end
+
+    # Like #annotate, but modifies relation in place.
+    def annotate!(*args) # :nodoc:
+      self.annotate_values += args
+      self
+    end
+
     # Returns the Arel object associated with the relation.
     def arel(aliases = nil) # :nodoc:
       @arel ||= build_arel(aliases)
@@ -1004,6 +1024,7 @@ module ActiveRecord
         arel.distinct(distinct_value)
         arel.from(build_from) unless from_clause.empty?
         arel.lock(lock_value) if lock_value
+        arel.comment(*annotate_values) unless annotate_values.empty?
 
         arel
       end
diff --git a/activerecord/lib/arel/nodes.rb b/activerecord/lib/arel/nodes.rb
index 2f6dd9bc45..f994754620 100644
--- a/activerecord/lib/arel/nodes.rb
+++ b/activerecord/lib/arel/nodes.rb
@@ -61,6 +61,8 @@ require "arel/nodes/outer_join"
 require "arel/nodes/right_outer_join"
 require "arel/nodes/string_join"
 
+require "arel/nodes/comment"
+
 require "arel/nodes/sql_literal"
 
 require "arel/nodes/casted"
diff --git a/activerecord/lib/arel/nodes/comment.rb b/activerecord/lib/arel/nodes/comment.rb
new file mode 100644
index 0000000000..237ff27e7e
--- /dev/null
+++ b/activerecord/lib/arel/nodes/comment.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module Arel # :nodoc: all
+  module Nodes
+    class Comment < Arel::Nodes::Node
+      attr_reader :values
+
+      def initialize(values)
+        super()
+        @values = values
+      end
+
+      def initialize_copy(other)
+        super
+        @values = @values.clone
+      end
+
+      def hash
+        [@values].hash
+      end
+
+      def eql?(other)
+        self.class == other.class &&
+          self.values == other.values
+      end
+      alias :== :eql?
+    end
+  end
+end
diff --git a/activerecord/lib/arel/nodes/delete_statement.rb b/activerecord/lib/arel/nodes/delete_statement.rb
index a419975335..56249b2bad 100644
--- a/activerecord/lib/arel/nodes/delete_statement.rb
+++ b/activerecord/lib/arel/nodes/delete_statement.rb
@@ -3,7 +3,7 @@
 module Arel # :nodoc: all
   module Nodes
     class DeleteStatement < Arel::Nodes::Node
-      attr_accessor :left, :right, :orders, :limit, :offset, :key
+      attr_accessor :left, :right, :orders, :limit, :offset, :key, :comment
 
       alias :relation :left
       alias :relation= :left=
@@ -18,16 +18,18 @@ module Arel # :nodoc: all
         @limit = nil
         @offset = nil
         @key = nil
+        @comment = nil
       end
 
       def initialize_copy(other)
         super
         @left  = @left.clone if @left
         @right = @right.clone if @right
+        @comment = @comment.clone if @comment
       end
 
       def hash
-        [self.class, @left, @right, @orders, @limit, @offset, @key].hash
+        [self.class, @left, @right, @orders, @limit, @offset, @key, @comment].hash
       end
 
       def eql?(other)
@@ -37,7 +39,8 @@ module Arel # :nodoc: all
           self.orders == other.orders &&
           self.limit == other.limit &&
           self.offset == other.offset &&
-          self.key == other.key
+          self.key == other.key &&
+          self.comment == other.comment
       end
       alias :== :eql?
     end
diff --git a/activerecord/lib/arel/nodes/insert_statement.rb b/activerecord/lib/arel/nodes/insert_statement.rb
index d28fd1f6c8..8430dd23da 100644
--- a/activerecord/lib/arel/nodes/insert_statement.rb
+++ b/activerecord/lib/arel/nodes/insert_statement.rb
@@ -3,7 +3,7 @@
 module Arel # :nodoc: all
   module Nodes
     class InsertStatement < Arel::Nodes::Node
-      attr_accessor :relation, :columns, :values, :select
+      attr_accessor :relation, :columns, :values, :select, :comment
 
       def initialize
         super()
@@ -11,6 +11,7 @@ module Arel # :nodoc: all
         @columns  = []
         @values   = nil
         @select   = nil
+        @comment  = nil
       end
 
       def initialize_copy(other)
@@ -18,10 +19,11 @@ module Arel # :nodoc: all
         @columns = @columns.clone
         @values =  @values.clone if @values
         @select =  @select.clone if @select
+        @comment =  @comment.clone if @comment
       end
 
       def hash
-        [@relation, @columns, @values, @select].hash
+        [@relation, @columns, @values, @select, @comment].hash
       end
 
       def eql?(other)
@@ -29,7 +31,8 @@ module Arel # :nodoc: all
           self.relation == other.relation &&
           self.columns == other.columns &&
           self.select == other.select &&
-          self.values == other.values
+          self.values == other.values &&
+          self.comment == other.comment
       end
       alias :== :eql?
     end
diff --git a/activerecord/lib/arel/nodes/select_core.rb b/activerecord/lib/arel/nodes/select_core.rb
index 5df6ac8412..b6154b7ff4 100644
--- a/activerecord/lib/arel/nodes/select_core.rb
+++ b/activerecord/lib/arel/nodes/select_core.rb
@@ -3,7 +3,7 @@
 module Arel # :nodoc: all
   module Nodes
     class SelectCore < Arel::Nodes::Node
-      attr_accessor :projections, :wheres, :groups, :windows
+      attr_accessor :projections, :wheres, :groups, :windows, :comment
       attr_accessor :havings, :source, :set_quantifier, :optimizer_hints
 
       def initialize
@@ -18,6 +18,7 @@ module Arel # :nodoc: all
         @groups          = []
         @havings         = []
         @windows         = []
+        @comment         = nil
       end
 
       def from
@@ -39,12 +40,13 @@ module Arel # :nodoc: all
         @groups      = @groups.clone
         @havings     = @havings.clone
         @windows     = @windows.clone
+        @comment     = @comment.clone if @comment
       end
 
       def hash
         [
           @source, @set_quantifier, @projections, @optimizer_hints,
-          @wheres, @groups, @havings, @windows
+          @wheres, @groups, @havings, @windows, @comment
         ].hash
       end
 
@@ -57,7 +59,8 @@ module Arel # :nodoc: all
           self.wheres == other.wheres &&
           self.groups == other.groups &&
           self.havings == other.havings &&
-          self.windows == other.windows
+          self.windows == other.windows &&
+          self.comment == other.comment
       end
       alias :== :eql?
     end
diff --git a/activerecord/lib/arel/nodes/update_statement.rb b/activerecord/lib/arel/nodes/update_statement.rb
index cfaa19e392..015bcd7613 100644
--- a/activerecord/lib/arel/nodes/update_statement.rb
+++ b/activerecord/lib/arel/nodes/update_statement.rb
@@ -3,7 +3,7 @@
 module Arel # :nodoc: all
   module Nodes
     class UpdateStatement < Arel::Nodes::Node
-      attr_accessor :relation, :wheres, :values, :orders, :limit, :offset, :key
+      attr_accessor :relation, :wheres, :values, :orders, :limit, :offset, :key, :comment
 
       def initialize
         @relation = nil
@@ -13,16 +13,18 @@ module Arel # :nodoc: all
         @limit    = nil
         @offset   = nil
         @key      = nil
+        @comment  = nil
       end
 
       def initialize_copy(other)
         super
         @wheres = @wheres.clone
         @values = @values.clone
+        @comment = @comment.clone if @comment
       end
 
       def hash
-        [@relation, @wheres, @values, @orders, @limit, @offset, @key].hash
+        [@relation, @wheres, @values, @orders, @limit, @offset, @key, @comment].hash
       end
 
       def eql?(other)
@@ -33,7 +35,8 @@ module Arel # :nodoc: all
           self.orders == other.orders &&
           self.limit == other.limit &&
           self.offset == other.offset &&
-          self.key == other.key
+          self.key == other.key &&
+          self.comment == other.comment
       end
       alias :== :eql?
     end
diff --git a/activerecord/lib/arel/select_manager.rb b/activerecord/lib/arel/select_manager.rb
index 32286b67f4..4e9f527235 100644
--- a/activerecord/lib/arel/select_manager.rb
+++ b/activerecord/lib/arel/select_manager.rb
@@ -244,6 +244,15 @@ module Arel # :nodoc: all
       @ctx.source
     end
 
+    def comment(*values)
+      @ctx.comment = Nodes::Comment.new(values)
+      self
+    end
+
+    def comment_node
+      @ctx.comment
+    end
+
     private
       def collapse(exprs)
         exprs = exprs.compact
diff --git a/activerecord/lib/arel/tree_manager.rb b/activerecord/lib/arel/tree_manager.rb
index 0476399618..326c4f995c 100644
--- a/activerecord/lib/arel/tree_manager.rb
+++ b/activerecord/lib/arel/tree_manager.rb
@@ -36,6 +36,11 @@ module Arel # :nodoc: all
         @ast.wheres << expr
         self
       end
+
+      def comment(*values)
+        @ast.comment = Nodes::Comment.new(values)
+        self
+      end
     end
 
     attr_reader :ast
diff --git a/activerecord/lib/arel/visitors/depth_first.rb b/activerecord/lib/arel/visitors/depth_first.rb
index 109afb7402..d696edc507 100644
--- a/activerecord/lib/arel/visitors/depth_first.rb
+++ b/activerecord/lib/arel/visitors/depth_first.rb
@@ -181,6 +181,10 @@ module Arel # :nodoc: all
           visit o.limit
         end
 
+        def visit_Arel_Nodes_Comment(o)
+          visit o.values
+        end
+
         def visit_Array(o)
           o.each { |i| visit i }
         end
diff --git a/activerecord/lib/arel/visitors/dot.rb b/activerecord/lib/arel/visitors/dot.rb
index 37803ce0c0..ecc386de07 100644
--- a/activerecord/lib/arel/visitors/dot.rb
+++ b/activerecord/lib/arel/visitors/dot.rb
@@ -234,6 +234,10 @@ module Arel # :nodoc: all
         end
         alias :visit_Set :visit_Array
 
+        def visit_Arel_Nodes_Comment(o)
+          visit_edge(o, "values")
+        end
+
         def visit_edge(o, method)
           edge(method) { visit o.send(method) }
         end
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