Mongoose return a empty array

Mongoose return a empty array when i am trying to use find() function.Can any one help me with this please?

users.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;    
mongoose.Promise = global.Promise;

const userSchema = new Schema({
    name: String    
});
const user = mongoose.model('user', userSchema, 'users');    
module.exports = user;

test_helper.js

   const mongoose = require('mongoose');

    mongoose.connect('mongodb://localhost/user_test');

    before((done)=>{
        mongoose.connection.once('open',()=>{
            console.log('Connected');
        }).on('error', ()=>{
            console.log('Error');
        });
        done();
    });

    beforeEach((done)=>{
        mongoose.connection
                .collections
                .users
                .drop(()=>{
            done();
        });
     });

create_user.js

const assert = require('assert');
const User = require('../src/user');


describe('Create a user',()=>{
    it('it should create a new user',(done)=>{
        let joe = new User({
            name : "Joe"
        });
        joe.save().then(()=>{
            assert(!joe.isNew);
            done();
        });
    });
})

reading-users.js

const assert = require('assert');
const User = require('../src/user');


describe('This will read all users', () => {
    before((done) => {
        joe = new User({name:'joe'});
        joe.save().then(() => done());
    });
    it('It Should find all users named joe', (done) => {
        User.findOne({name:"Joe"})
            .then((foo)=>{
                //assert(users[0]._id.toString()===joe._id.toString());
                console.log(foo);
                console.log(joe);
                done();
        });
    });
});

=============================log==========================================

(node:3324) DeprecationWarning: current URL string parser is deprecated, and will be removed in a future version. To use the new parser, pass option { useNewUrlParser: true } to MongoClient.connect. Create a user Connected √ it should create a new user (274ms)

This will read all users [] { _id: 5bfa98eb6bd05d0cfc5d639d, name: 'joe', __v: 0 } √ It Should find all users named joe

2 passing (1s)