Issue指派给为0的时候迁移

db/migrate/20161111064007_update_assgin_to_id.rb

@@ -0,0 +1,8 @@
+class UpdateAssginToId < ActiveRecord::Migration
+  def up
+    Issue.find_by_sql("update `issues` set assigned_to_id = null where assigned_to_id = 0;")
+  end
+
+  def down
+  end
+end