请教mongoose分组统计该怎么写呢

假如我有如下两个Collection：颜色和花，我要按颜色统计，对应的花的数量应该怎么写呢？用mongoose。

var color = new mongoose.Schema({
    color: {type: String}
});
var flower = new mongoose.Schema({
	name:{type:String},
    color: {type: mongoose.Schema.ObjectId,ref:'Color'}
});