module.exports = Book= mongoose.model('Book', new Schema({
 name : String,
 checked_in : Boolean
},{ collection : 'Book' }));

var action = new Schema({
 checked_in: Boolean,
});

module.exports = Activity = mongoose.model('Activity', new Schema({
 book_id: String,
 actions: [action]
},{ collection : 'Activity' }));

exports.update = function(req, res){
    return Book.findById(req.params.id, function(err, book) {
       var activity = new Activity({book_id: book.id});
       activity.actions.push({
           checked_in: req.body.checked_in,
       });

       Activity.update({ book_id: book.id}, activity.toObject(), { upsert: true }));

       book.checked_in = req.body.checked_in;
       return device.save(function(err) {
           return res.send(book);
       });
    });
};

module.exports = Activity = mongoose.model('Activity', new Schema({
  book_id: Schema.ObjectId,
  actions: [new Schema({
    checked_in: Boolean,
    last_user: String
  })]
},{ collection : 'Activity' }));

exports.update = function(req, res){
  // TODO: Check for undefined.
  return book.findById(req.params.id, function(err, book) {
    if(!err) {
      // Update the book.
      book.checked_in = req.body.checked_in;
      book.last_user = req.body.last_user;
      book.save();

      // If there's no associated activity for the book, create one.
      // Otherwise update and push new activity to the actions array.
      Activity.findById(book._id, function (err, activity) {
        activity.actions.push({
          checked_in: req.body.checked_in,
          last_user: req.body.last_user
        })

        activity.save();
      });
    }
  });
};

{
    book_id: "5058c5ddeeb0a3aa253cf9d4",
    actions: [
        { checked_in: true, last_user: 'ralph' },
        { checked_in: true, last_user: 'gonzo' },
        { checked_in: true, last_user: 'animal' }
    ]
}

Activity.findOne({book_id: book._id}, function (err, activity) {
  if (!activity) {
    // No Activity doc for the book yet, create one.
    activity = new Activity({book_id: book._id});
  }
  activity.actions.push({
    checked_in: req.body.checked_in,
    last_user: req.body.last_user
  });

  activity.save();
});