опитайте това
SubSchema = new Schema({
_member: {type: ObjectId, ref: 'Member'},
members: [{type: ObjectId, ref: 'Member'}],
created: { type: Date, default: Date.now }
});
var SubModel = mongoose.model('SubModel', SubSchema);//add
MainSchema = new Schema({
_member: {type: ObjectId, ref: 'Member'},
subs: [SubSchema],
members: [{type: ObjectId, ref: 'Member'}],
created: { type: Date, default: Date.now }
});
var MainModel = mongoose.model('MainModel', MainSchema);
MainModel.find({})
.sort('created', -1)
.limit(25)
.populate("_member")
.populate("subs._member")
.exec(function(err, mains){
//add
SubModel.populate(mains,{
path:'subs.members'
},function(err,mains){
//mains[0].subs[0].members - is not empty
});
});