mongoose的populate的联合查询,双向联合查询

Question

const Schema = mongoose.Schema;
var Animal = new Schema({
name:String
});

var Category = new Schema({
category: [{type: Schema.Types.ObjectId, ref: 'Animal'}]
});

var Animal = mongoose.model('animal', Animal);
var Category = mongoose.model('category', Category);

var animal = new Animal({

name:'dog'  

});

animal.save(function (err) {

if (err) return handleError(err);  

var category = new Category({

    category: animal._id  

});
category.save();
});

Category.find().populate('category')

通过Category可以查出关联的category字段对应Animal是什么 ,但是我该怎么想通过Animal查询出对应的Category呢.