字段ObjectId ref为null mongodb

Question

我有这个模型：

const schema = new Schema({
    title: {type: String, required: true},
    description: {type: String},
    absense: {type: Schema.Types.ObjectId, ref: 'Abscense'},
    contract: {type: Schema.Types.ObjectId, ref: 'Contract'},
    date: {type: Date},
    type: {type: String},
    discount_day: {type: Boolean, default: true},
    discount_vt: {type: Boolean, default: true},
    discount_vr: {type: Boolean, default: true},
    crm: {type: String},
    doctor_name: {type: String},
    child: {type: String}
});

但是在某些情况下，缺少字段是一个空值。可以将其设置为null吗？