Push to array within a model using Mongo/Express/Angular/Node

I have a model called Patients and a model called Drugs. Each patient can be taking multiple drugs.

I'm using ng-repeat to loop through the patients and display. Each patient displays a select box containing the list of available drugs. I am able to log the id of the selected drug to the console, but I can't seem to successfully push it to the model.

<Node Models >

Patient.model.js

    module.exports = function (mongoose, name) {
        var mongoose = require('mongoose'),
        Schema = mongoose.Schema,
        ObjectId = Schema.ObjectId;

        var schema = mongoose.Schema({
            name: String,
            drugs: [String],
            tests: [String]
        });

        mongoose.model(name, schema);
    };

Drug.model.js

    module.exports = function (mongoose, name) {
        var mongoose = require('mongoose'),
        Schema = mongoose.Schema,
        ObjectId = Schema.ObjectId;

        var schema = mongoose.Schema({
            name: String
        });

        mongoose.model(name, schema);
    };

< Node Controller >

Patient.controller.js

var mongoose = require('mongoose'),
    Patient = mongoose.model('Patient');

exports.create = function (req, res) {
  var patient = new Patient(req.body);
  patient.save(function (err) {
    if (err) {
      res.send(400, err);
    } else {
      res.send(patient);
    }
  });
};

exports.read = function (req, res) {
  Patient.findById(req.params.id, function (err, patient) {
    if (err) {
      res.send(400, err);
    } else if (!patient) {
      res.send(404);
    } else {
      res.send(patient);
    }
  });
};

exports.update = function (req, res) {
  var id = req.params.id,
      data = req.body;

  delete data._id; // Just in case...

  Patient.findByIdAndUpdate(id, data, function (err) {
    if (err) {
      res.send(400, err);
    } else {
      res.send({success: true, msg: 'saved'});
    }
  });
};

exports.del = function (req, res) {
  var id = req.params.id;

  Patient.findById(id, function (err, patient) {
    if (err) {
      res.send(400, err);
    } else if (!patient) {
      res.send(404);
    } else {
      patient.remove(function (err) {
        if (err) {
          res.send(400, err);
        } else {
          res.send({success: true, msg: 'removed'});
        }
      });
    }
  });
};

exports.search = function (req, res) {
  Patient.find(req.query, function (err, patients) {
    if (err) {
      res.send(400, err);
    } else {
      res.send(patients);
    }
  });
};

< Angular Controller >

patientController.js

  angular
    .module('dpMean.patient')
    .controller('PatientCtrl', function ($scope, Restangular, $log) {
      'use strict';
      var Patient = Restangular.all('patient');
      Patient.getList()
        .then(function (patients) {
          $scope.patients = patients;
        })
        .catch(function (err) {
          $log.error(err);
        });
      var Drug = Restangular.all('drug');
      Drug.getList()
        .then(function (drugs) {
          $scope.drugs = drugs;
        })
        .catch(function (err) {
          $log.error(err);
        });

        /* $scope.add and $scope.delete live here */

      $scope.select = function(id, pid){
        console.log(id);

        var patientDrugs = Restangular.one("patient", pid);
        var array = patientDrugs;
        console.log(array);
        var newarray = array.push(id);
        var update = {
          drugs: newarray
        }
        console.log(patientDrugs);
        patientDrugs.put(update);
      };
    });

< Angular Template >

<div>
    <h3>Patients</h3>
    <ul class="unstyled">
      <li class="patient-item" ng-repeat="patient in patients">
        <h4>{{patient.name}}</h4>
        <label>Add Drug</label>
        <select ng-options="drug.name for drug in drugs" ng-model="drug" ng-change="select(drug._id, patient._id)">
        </select>
        <ul>
            <li ng-repeat="drug in patient.drugs">
                {{drug}}
            </li>
        </ul>
        {{ patient.drugs}}

        <hr />
        <a ng-click="delete(patient._id)">[Remove Patient]</a>

      </li>
      <li class="patient-item">
        <form ng-submit="add()">
          <input type="text" placeholder="New item..." ng-model="name"/>
          <button type="submit" ng-disabled="posting || !name">Add</button>
        </form>
      </li>
    </ul>
</div>

Desired behavior: When user selects a drug from the select box, the id of the selected drug will be added to patient.{patientID}.drugs, which is an array.

Thanks so much for any help you can give.