var mongoose = require('mongoose')
var Schema = mongoose.Schema
var personSchema = Schema({
  _id     : Number,
  name    : String,
  age     : Number,
  stories : [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});
var storySchema = Schema({
  _creator : { type: Number, ref: 'Person' },
  title    : String,
  fans     : [{ type: Number, ref: 'Person' }]
});
var Story  = mongoose.model('Story', storySchema);
var Person = mongoose.model('Person', personSchema);  
\n``` 
上述代码中，为什么storySchema中的_creator字段为什么   之前要加   下划线？