implements AR::Relation#explain

This is a first implementation, EXPLAIN is highly
dependent on the database and I have made some
compromises.

On one hand, the method allows you to run the most
common EXPLAIN and that's it. If you want EXPLAIN
ANALYZE in PostgreSQL you need to do it by hand.

On the other hand, I've tried to construct a string
as close as possible to the ones built by the
respective shells. The rationale is that IMO the
user should feel at home with the output and
recognize it at first sight. Per database.

I don't know whether this implementation is going
to work well. Let's see whether people like it.

1 files changed, 16 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 3baf9b3f49..f0891440a6 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -143,6 +143,22 @@ module ActiveRecord
         super
     end
 
+    def explain
+      queries = []
+      callback = lambda do |*args|
+        payload = args.last
+        queries << payload[:sql] unless payload[:exception] || %w(SCHEMA EXPLAIN).include?(payload[:name])
+      end
+
+      ActiveSupport::Notifications.subscribed(callback, "sql.active_record") do
+        to_a
+      end
+
+      queries.map do |sql|
+        @klass.connection.explain(sql)
+      end.join
+    end
+
     def to_a
       return @records if loaded?