add unique constraint to employee

db/migrate/20200114115657_add_unique_constraint.rb

@@ -0,0 +1,9 @@
+class AddUniqueConstraint < ActiveRecord::Migration[5.1]
+  def change
+    remove_index :employees, :app_id
+    remove_index :employees, :app_token
+
+    add_index :employees, :app_id, unique: true
+    add_index :employees, :app_token, unique: true
+  end
+end