课程学生列表显示太慢，需要对表加索引

db/migrate/20150505023015_add_index_to_homeworkattach_bid_id.rb

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+class AddIndexToHomeworkattachBidId < ActiveRecord::Migration
+  def change
+    add_index(:homework_attaches,:bid_id)
+  end
+end

db/migrate/20150505023127_add_index_to_studentforcourse_student_id.rb

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+class AddIndexToStudentforcourseStudentId < ActiveRecord::Migration
+  def change
+    add_index(:students_for_courses,:student_id)
+  end
+end

db/migrate/20150505023452_add_index_to_studentforcourse_course_id.rb

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+class AddIndexToStudentforcourseCourseId < ActiveRecord::Migration
+  def change
+    add_index(:students_for_courses,:course_id)
+  end
+end

db/migrate/20150505025003_add_index_to_homeworkforcourse_course_id.rb

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+class AddIndexToHomeworkattachCourseId < ActiveRecord::Migration
+  def change
+    add_index(:homework_for_courses,:course_id)
+  end
+end

db/migrate/20150505025537_add_index_to_homeworkforcourse_bid_id.rb

@@ -0,0 +1,5 @@
+class AddIndexToHomeworkforcourseBidId < ActiveRecord::Migration
+  def change
+    add_index(:homework_for_courses,:bid_id)
+  end
+end