参考：https://api.rubyonrails.org/v5.1/classes/ActiveRecord/EagerLoadPolymorphicError.html
https://api.rubyonrails.org/v5.1/classes/ActiveRecord/QueryMethods.html#method-i-preload
报错代码：
@orders = Order.all
@orders = @orders.joins(:user).merge(User.with_level(params[:level])) if params[:level].in? Order.level.values
@orders = @orders.includes(:user, :orderable).recent.paginate(page: params[:page], per_page: 30)
解决问题：
@orders = Order.all
@orders = @orders.joins(:user).merge(User.with_level(params[:level])) if params[:level].in? Order.level.values
@orders = @orders.preload(:user, :orderable).recent.paginate(page: params[:page], per_page: 30)
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