问Rails 5迁移数据类型问题

EN

class AddAdminRefToCart < ActiveRecord::Migration[5.1]
  def change
    add_reference :carts, :admin, foreign_key: true
  end
end

rake db:migrate

== 20171129152942 AddAdminRefToCart: migrating ================================
-- add_reference(:carts, :admin, {:foreign_key=>true})
rake aborted!
StandardError: An error has occurred, all later migrations canceled:

Column `admin_id` on table `carts` has a type of `bigint(20)`.
This does not match column `id` on `admins`, which has type `int(11)`.
To resolve this issue, change the type of the `admin_id` column on `carts` to be :integer. (For example `t.integer admin_id`).