merge_collection_mongodb.js

db.authors.insert([
	{
		_id: 'a1',
		name: { first: 'orlando', last: 'becerra' },
		age: 27
	},
	{
		_id: 'a2',
		name: { first: 'mayra', last: 'sanchez' },
		age: 21
	}
]);
db.categories.insert([
	{
		_id: 'c1',
		name: 'sci-fi'
	},
	{
		_id: 'c2',
		name: 'romance'
	}
]);
db.books.insert([
	{
		_id: 'b1',
		name: 'Groovy Book',
		category: 'c1',
		authors: ['a1']
	},
	{
		_id: 'b2',
		name: 'Java Book',
		category: 'c2',
		authors: ['a1','a2']
	},
]);
db.lendings.insert([
	{
		_id: 'l1',
		book: 'b1',
		date: new Date('01/01/11'),
		lendingBy: 'jose'
	},
	{
		_id: 'l2',
		book: 'b1',
		date: new Date('02/02/12'),
		lendingBy: 'maria'
	}
]);


db.books.find().forEach(
	function (newBook) {
		newBook.category = db.categories.findOne( { "_id": newBook.category } );
		newBook.lendings = db.lendings.find( { "book": newBook._id  } ).toArray();
		newBook.authors = db.authors.find( { "_id": { $in: newBook.authors }  } ).toArray();
		db.booksReloaded.insert(newBook);
	}
);
db.booksReloaded.find().pretty()