Model

.find(query)

.populate({

  path: 'key_with_ref',

  model: 'model_name',

  select: { 'field_name': 1,'field_name':1},

})

-OR-

​

Model

.find(query)

.populate({

  path: 'key_with_ref',

  model: 'model_name',

  select: 'field_name, field_name',

})