mongodb自动递增

这是一个示例，如何在Mongoose中实现自动增量字段：

var CounterSchema = Schema({    _id: {type: String, required: true},    seq: { type: Number, default: 0 }});var counter = mongoose.model('counter', CounterSchema);var entitySchema = mongoose.Schema({    testvalue: {type: String}});entitySchema.pre('save', function(next) {    var doc = this;    counter.findByIdAndUpdate({_id: 'entityId'}, {$inc: { seq: 1} }, function(error, counter)   {        if(error) return next(error);        doc.testvalue = counter.seq;        next();    });});