Sparse indexes in Mongo with Mongoose

test.js

// Assumes you've already somehow configured your db access.  I left that part out

var mongoose = require("mongoose")

var es = new mongoose.Schema({
      'firstname': { type: String, required: true }, 
      'lastname': { type: String, required: true }, 
      'email': { type: String, required: true, index: { unique: true, sparse: true }, validate: /\b[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}\b/ },
      'departmentId': { type: mongoose.Schema.ObjectId, required: true },
      'enddate': String,
      'active': { type: Boolean, default: true }}) 

var EmployeeSchemaModel = mongoose.model('employees', es)

var DepartmentSchema = new mongoose.Schema({
     'name': { type: String, required: true, index: { unique: true } },
     'employees' : [ es ]
})
                                                            
var DepartmentSchemaModel = mongoose.model('departments', DepartmentSchema)

var a = DepartmentSchemaModel.create({name: "a"}, function(err, d) {
  if (err) console.log("got error on first");

  DepartmentSchemaModel.create({name: "b"}, function(err, e) {
    console.log("on #2")
    console.log(err) // we don't get this error
  })
})