var a = new Schema({ name: ‘’, b_id: {type: Schema.ObjectId, ref: ‘b’} });

var b = new Schema({ name: ‘’, });

a: {name: ‘1’, b_id: ‘…’} {name: ‘2’, b_id: ‘…’} {name: ‘3’, b_id: ‘…’}

b: {name: ‘b1’} {name: ‘b2’} {name: ‘b3’}

a.find({}) .populate({ path: ‘b_id’, match: {‘name’: ‘b1’} }) .exec(callback);

怎么实现Inner Join ??