为student_work表添加索引

为homework_comments添加索引
大幅提高查询速度

db/migrate/20151130031446_add_index_to_homework_commons.rb

@@ -0,0 +1,5 @@
+class AddIndexToHomeworkCommons < ActiveRecord::Migration
+  def change
+    add_index :homework_commons, [:course_id, :id]
+  end
+end

db/migrate/20151130033906_add_index_to_student_works.rb

@@ -0,0 +1,5 @@
+class AddIndexToStudentWorks < ActiveRecord::Migration
+  def change
+    add_index :student_works, [:homework_common_id, :user_id]
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20151127011351) do
+ActiveRecord::Schema.define(:version => 20151130033906) do
 
   create_table "activities", :force => true do |t|
     t.integer  "act_id",                                  :null => false
@@ -736,6 +736,8 @@ ActiveRecord::Schema.define(:version => 20151127011351) do
     t.integer  "anonymous_comment", :default => 0
   end
 
+  add_index "homework_commons", ["course_id", "id"], :name => "index_homework_commons_on_course_id_and_id"
+
   create_table "homework_detail_manuals", :force => true do |t|
     t.float    "ta_proportion"
     t.integer  "comment_status"
@@ -961,6 +963,7 @@ ActiveRecord::Schema.define(:version => 20151127011351) do
     t.integer  "course_group_id",   :default => 0
   end
 
+  add_index "members", ["course_id"], :name => "index_members_on_course_id"
   add_index "members", ["project_id"], :name => "index_members_on_project_id"
   add_index "members", ["user_id", "project_id", "course_id"], :name => "index_members_on_user_id_and_project_id", :unique => true
   add_index "members", ["user_id"], :name => "index_members_on_user_id"
@@ -1506,6 +1509,8 @@ ActiveRecord::Schema.define(:version => 20151127011351) do
     t.boolean  "is_test",                                       :default => false
   end
 
+  add_index "student_works", ["homework_common_id", "user_id"], :name => "index_student_works_on_homework_common_id_and_user_id"
+
   create_table "student_works_evaluation_distributions", :force => true do |t|
     t.integer  "student_work_id"
     t.integer  "user_id"