mongoose怎么populate一个对象数组里的某个属性字段

ASchema=newmongoose.Schema({name:String,B:[BSchema]});varBSchema=newSchema({name:String});varCSchema=newSchema({name:String,B:{type:Schema.ObjectId,ref:'BSchema'}});//ThisisOK,butnotonetimemongoosesearch:CSchema.find({name:'C_xxx'}).exec(function(err,docC){docC.forEach(function(o){varBId=o.B;A.findOne({'B._id':BId},{'B.$':1}).exec(function(err,docA){varBname=docA.B[0].name;varCname=docA.name;});});});//wronginpopulate:CSchema.find({name:'C_xxx'}).populate('ASchema.B').exec(function(err,docC){docC.forEach(function(o){varBname=o.B.name;//ERROR:o.BisaobjectId,o.B.nameisundefined//Anamecan'tfind});});