mongodb - Mongo group and push: pushing all fields

Question

Is there an easy way to "$push" all fields of a document? For example:

Say I have a Mongo collection of books:

{author: "tolstoy", title:"war & peace", price:100, pages:800}
{author: "tolstoy", title:"Ivan Ilyich", price:50,  pages:100}

I'd like to group them by author - for each author, list his entire book objects:

{ author: "tolstoy",
  books: [
     {author: "tolstoy", title:"war & peace", price:100, pages:800}
     {author: "tolstoy", title:"Ivan Ilyich", price:50,  pages:100}
  ]
}

I can achieve this by explicitly pushing all fields:

{$group: {
     _id: "$author",
     books:{$push: {author:"$author", title:"$title", price:"$price", pages:"$pages"}},
}}

But is there any shortcut, something in the lines of:

// Fictional syntax...
{$group: {
    _id: "$author",
    books:{$push: "$.*"},
}}

Answer 1

You can use $$ROOT

{ $group : {
            _id : "$author",
            books: { $push : "$$ROOT" }
        }}

Found here: how to use mongodb aggregate and retrieve entire documents