add the query to AR::Relation#explain output

Rationale: this is more readable if serveral queries
are involved in one call. Also, it will be possible
to let AR log EXPLAINs automatically in production
mode, where queries are not even around.

5 files changed, 17 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index f0891440a6..0c32ad5139 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 require 'active_support/core_ext/object/blank'
 require 'active_support/core_ext/module/delegation'
 
@@ -155,8 +156,8 @@ module ActiveRecord
       end
 
       queries.map do |sql|
-        @klass.connection.explain(sql)
-      end.join
+        "EXPLAIN for: #{sql}\n#{@klass.connection.explain(sql)}"
+      end.join("\n")
     end
 
     def to_a
diff --git a/activerecord/test/cases/adapters/mysql2/explain_test.rb b/activerecord/test/cases/adapters/mysql2/explain_test.rb
index 8ea777b72b..68ed361aeb 100644
--- a/activerecord/test/cases/adapters/mysql2/explain_test.rb
+++ b/activerecord/test/cases/adapters/mysql2/explain_test.rb
@@ -9,12 +9,15 @@ module ActiveRecord
 
         def test_explain_for_one_query
           explain = Developer.where(:id => 1).explain
+          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers`  WHERE `developers`.`id` = 1), explain
           assert_match %(developers | const), explain
         end
 
         def test_explain_with_eager_loading
           explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers`  WHERE `developers`.`id` = 1), explain
           assert_match %(developers | const), explain
+          assert_match %(EXPLAIN for: SELECT `audit_logs`.* FROM `audit_logs`  WHERE `audit_logs`.`developer_id` IN (1)), explain
           assert_match %(audit_logs | ALL), explain
         end
       end
diff --git a/activerecord/test/cases/adapters/postgresql/explain_test.rb b/activerecord/test/cases/adapters/postgresql/explain_test.rb
index 0d599ed37f..0b61f61572 100644
--- a/activerecord/test/cases/adapters/postgresql/explain_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/explain_test.rb
@@ -9,6 +9,7 @@ module ActiveRecord
 
         def test_explain_for_one_query
           explain = Developer.where(:id => 1).explain
+          assert_match %(EXPLAIN for: SELECT "developers".* FROM "developers"  WHERE "developers"."id" = 1), explain
           assert_match %(QUERY PLAN), explain
           assert_match %(Index Scan using developers_pkey on developers), explain
         end
@@ -16,7 +17,9 @@ module ActiveRecord
         def test_explain_with_eager_loading
           explain = Developer.where(:id => 1).includes(:audit_logs).explain
           assert_match %(QUERY PLAN), explain
+          assert_match %(EXPLAIN for: SELECT "developers".* FROM "developers"  WHERE "developers"."id" = 1), explain
           assert_match %(Index Scan using developers_pkey on developers), explain
+          assert_match %(EXPLAIN for: SELECT "audit_logs".* FROM "audit_logs"  WHERE "audit_logs"."developer_id" IN (1)), explain
           assert_match %(Seq Scan on audit_logs), explain
         end
       end
diff --git a/activerecord/test/cases/adapters/sqlite3/explain_test.rb b/activerecord/test/cases/adapters/sqlite3/explain_test.rb
index e18892821d..b227bce680 100644
--- a/activerecord/test/cases/adapters/sqlite3/explain_test.rb
+++ b/activerecord/test/cases/adapters/sqlite3/explain_test.rb
@@ -9,12 +9,15 @@ module ActiveRecord
 
         def test_explain_for_one_query
           explain = Developer.where(:id => 1).explain
+          assert_match %(EXPLAIN for: SELECT "developers".* FROM "developers"  WHERE "developers"."id" = 1), explain
           assert_match(/(SEARCH )?TABLE developers USING (INTEGER )?PRIMARY KEY/, explain)
         end
 
         def test_explain_with_eager_loading
           explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(EXPLAIN for: SELECT "developers".* FROM "developers"  WHERE "developers"."id" = 1), explain
           assert_match(/(SEARCH )?TABLE developers USING (INTEGER )?PRIMARY KEY/, explain)
+          assert_match %(EXPLAIN for: SELECT "audit_logs".* FROM "audit_logs"  WHERE "audit_logs"."developer_id" IN (1)), explain
           assert_match(/(SCAN )?TABLE audit_logs/, explain)
         end
       end
diff --git a/railties/guides/source/active_record_querying.textile b/railties/guides/source/active_record_querying.textile
index ad12dca7e8..0f1f6eba4c 100644
--- a/railties/guides/source/active_record_querying.textile
+++ b/railties/guides/source/active_record_querying.textile
@@ -1287,6 +1287,7 @@ User.where(:id => 1).joins(:posts).explain
 may yield
 
 <plain>
+EXPLAIN for: SELECT `users`.* FROM `users` INNER JOIN `posts` ON `posts`.`user_id` = `users`.`id` WHERE `users`.`id` = 1
 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------------<plus>
 | id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------------<plus>
@@ -1302,6 +1303,7 @@ Active Record performs a pretty printing that emulates the one of the database
 shells. So, the same query running with the PostgreSQL adapter would yield instead
 
 <plain>
+EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE "users"."id" = 1
                                   QUERY PLAN
 ------------------------------------------------------------------------------
  Nested Loop Left Join  (cost=0.00..37.24 rows=8 width=0)
@@ -1324,12 +1326,15 @@ User.where(:id => 1).includes(:posts).explain
 yields
 
 <plain>
+EXPLAIN for: SELECT `users`.* FROM `users`  WHERE `users`.`id` = 1
 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------<plus>
 | id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------<plus>
 |  1 | SIMPLE      | users | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
 <plus>----<plus>-------------<plus>-------<plus>-------<plus>---------------<plus>---------<plus>---------<plus>-------<plus>------<plus>-------<plus>
 1 row in set (0.00 sec)
+
+EXPLAIN for: SELECT `posts`.* FROM `posts`  WHERE `posts`.`user_id` IN (1)
 <plus>----<plus>-------------<plus>-------<plus>------<plus>---------------<plus>------<plus>---------<plus>------<plus>------<plus>-------------<plus>
 | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
 <plus>----<plus>-------------<plus>-------<plus>------<plus>---------------<plus>------<plus>---------<plus>------<plus>------<plus>-------------<plus>