匿评分配增加唯一索引

db/migrate/20200109095357_add_uniq_index_to_evaluation_distribution.rb

@@ -0,0 +1,11 @@
+class AddUniqIndexToEvaluationDistribution < ActiveRecord::Migration[5.2]
+  def change
+    sql = %Q(delete from student_works_evaluation_distributions where (user_id, student_work_id) in
+            (select * from (select user_id, student_work_id from student_works_evaluation_distributions group by user_id, student_work_id having count(*) > 1) a)
+            and id not in (select * from (select min(id) from student_works_evaluation_distributions group by user_id, student_work_id having count(*) > 1 order by id) b))
+    ActiveRecord::Base.connection.execute sql
+
+    add_index :student_works_evaluation_distributions, [:student_work_id, :user_id], name: "index_on_student_work_id_and_user_id", unique: true
+    remove_index :student_works_evaluation_distributions, :user_id
+  end
+end