我有一个mongoose模型架构，设置如下。varorderSchema=newSchema({_invoices:[{type:Schema.ObjectId,ref:'invoice'},_discounts:[{type:Schema.ObjectId,ref:'discount'},_client:String});orderSchema.methods.totalInvoiced=function(cb){this.populate('_invoices',function(err,order){cb(err,_.reduce(_.pluck(order._invoices,