how to populate in mongoose?

var headSchema = new Schema({
  typename: String,
  headname: String,
  status: { type: Number, default: 1 }
});
var Head = mongoose.model("heads", headSchema);

var mgmtSchema = new Schema({
  headtype: String,
  type: { type: Schema.Types.ObjectId, ref: "Head" },
  datefrom: Date,
  dateto: Date,
  title: String,
  amount: Number,
  description: String,
  created_at: { type: Date, default: Date.now },
  updated_at: { type: Date, default: Date.now },
  status: Number
});

var HeadMgmt = mongoose.model("headmgmt", mgmtSchema);

HeadMgmt.find({})
  .populate({ path: "type", select: "headname" })
  .exec(function(err, result) {
    console.log(result);
  });

I have two schemas, also foreign key reference. The second schema has some value. I need to display the lists in one html table. How can print headname from head schema?

In mgmtSchema you need to change the ref value in type field from Head to heads, because you used headssetup your Head model like this:

var Head = mongoose.model("heads", headSchema);

So your mgmtSchema must be like this:

var mgmtSchema = new Schema({
  headtype: String,
  type: { type: Schema.Types.ObjectId, ref: "heads" },
  datefrom: Date,
  dateto: Date,
  title: String,
  amount: Number,
  description: String,
  created_at: { type: Date, default: Date.now },
  updated_at: { type: Date, default: Date.now },
  status: Number
});

The result of your current populate code will be something like this:

[
    {
        "_id": "5e13483dd62cd92e3c3e8dcd",
        "headtype": "headtype",
        "type": {
            "_id": "5e1347671b0bd339e0521139",
            "headname": "headname 1"
        },
        "title": "title",
        "amount": 123,
        "description": "description",
        "created_at": "2020-01-06T14:46:21.906Z",
        "updated_at": "2020-01-06T14:46:21.906Z",
        "__v": 0
    }
]