text: 
  var mongoose = require('mongoose');
  
  var animalSchema = require("../schemas/animal");
  
  animalSchema.index({ _id: 1 }, { sparse: true });
  
  var Animal = mongoose.model('Animal', animalSchema);
  
  Animal.on('index', function(error) {
    // "_id index cannot be sparse"
    console.log(error.message);
  });
  
  module.exports = Animal;