現在有個文章post的model使用cateId關聯Category類別目。但cateId並非_id。導致獲取時報錯。
請問只能透過_id去關聯嗎？如何使用Category下的非_id進行關聯？

var PostSchema = new mongoose.Schema({
    author: { type: mongoose.Schema.Types.ObjectId},
    title: String,
    content: String,
    imgUrl: String,
    pv: { type:Number, default:0}, 
    like: { type:Number, default:0}, 
    keep:  { type:Number, default:0}, 
    comments: { type:Number, default:0}, 
    cateId: { type:Number, ref:'Category'},
    type:  Number,
    create_date: { type: Date, default: Date.now }
},{
    versionKey: false 
})

var Post = mongoose.model('Post', PostSchema)