Mongoose，索引位置数组

我在文档中有一个位置数组，我想在该数组上添加 2dSpere 索引。那可能吗？

var LocationSchema = new Schema({
  type: { type: String, required: true },
  geometry: {
    type: { type: String, required: true },
    coordinates: { type: Array, required: true}
  },
  properties: Schema.Types.Mixed
});

// Collection to hold users
var UserSchema = new Schema({
    username: { type: String, required: true, unique: true },
    locations: [LocationSchema]
  },{ 
    versionKey: false
  }
);

如何在位置数组中的几何字段上添加 2DSphere 索引？

https://github.com/LearnBoost/mongoose/blob/master/examples/geospatial/geoJSONSchema.js#L19 https://github.com/LearnBoost/mongoose/blob/master/examples/geospatial/geoJSONSchema.js#L19

LocationSchema.index({ 'geometry' : '2dsphere' });