javascript - How to populate an array of sub `ref`ed documents mongoose? -

i'm working on mean stack app mongoose version 4.4.0 , having problem populating array of sub-documents. (couldn't find solutions elsewhere on so.)

the following illustrates structure of models:

visa model

var visaschema = new mongoose.schema({   from: {type: schema.types.objectid, ref: 'country'},   to: {type: schema.types.objectid, ref: 'country'},   requirements: [     {type: schema.types.objectid, ref: 'requirement'}   ] });  exports.model = mongoose.model('visa', visaschema); 

requirement model

var requirementschema = new mongoose.schema({    visa: {type: schema.types.objectid, ref: "visa"},    title: string,    body: string });  exports.model = mongoose.model('requirement', requirementschema); 

the above structures one-to-many relationship between visa , requirement.

problem

i not able populate requirements field in fact array of requirement ref objects.

this how i'm trying achieve it:

visa.find()    //.populate('from', null, {shortname: from})    //.populate('to', null, {shortname: to})    .populate('requirements')    .exec(function(err, result){       res.json(result);    }); 

whenever try populate requirements field, returns empty array on requirements key.

however without population, returns array of objectids:

[    {      "_id":"56b329878848eba00c0f5535",      "from"{           "_id":"56b2db1b57f9e71c1bfbadb9",           "name":"germany",           "shortname":"de",           "__v":0      },      "to":{           "_id":"56b2db0f57f9e71c1bfbadb8",           "name":"afghanistan",           "shortname":"afg",           "__v":0      },      "__v":3,      "requirements":  [           "56b331d764585e8c12c14dd1",            "56b336431ffc49bc19bf1c96",           "56b33675443f033c14e26d03"       ]    } ] 

what doing wrong?

any kind of appreciated.