// *models/user.js
 var userSchema = mongoose.Schema({
      email: {type: 'string'},
      password: {type: 'string'},
      posts: [{type: mongoose.Schema.Types.ObjectId, ref: 'Post'}]
    });
 module.exports = mongoose.model('User', userSchema);

// *models/post.js
var postSchema = mongoose.Schema({
  user : { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
  body: String,
  title: String
});

module.exports = mongoose.model('Post', postSchema);

Post = require('./models/post');
User = require('./models/user');

app.post('/api/posts',(req,res)=>{

  User.findOne({email: "valid_email"},(err, user)=>{

    var post = {
      title: "Post 1 Title",
      body: "Post 1 body",
      user: user._id
    }
    Post.create(post, (err, post)=>{
      res.send(post);
   });
  });
});

app.get('/post', (req,res)=>{
  Post.findOne(title: "Post 1 title")
 .populate('user')
 .exec(function (err, user) {
   if (err) throw err;
   console.log(user._id, user.email);
   res.end();
 });
});