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:{$push: "$.*"},
}}

score 177 · Accepted Answer · edited May 23 '17 at 12:02

177

You can use $$ROOT

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

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

edited May 23 '17 at 12:02

Community

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1

answered Apr 17 '15 at 08:58

Jurjen Ladenius

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how to remove spesific field, exc I want to remove price field ? – silvia zulinka Jun 30 '21 at 14:43

Neil Lunn · Answer 2 · 2014-03-03T15:41:03.683

11

Actually you cant achieve what you are saying at all, you need $unwind

db.collection.aggregate([
    {$unwind: "$books"},

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

That is how you deal with arrays in aggregation.

And what you are looking for to shortcut typing all of the fields does not exist, yet.

But specifically because of what you have to do then you could not do that anyway as you are in a way, reshaping the document.

edited Mar 03 '14 at 15:41

answered Mar 03 '14 at 15:34

Neil Lunn

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Thanks very much for replying, but my query works (tested on mongo 2.4.9). Maybe my description wasn't clear - apologies if thats' the case. I start off with a flat Mongo collection of books (totally "un-wound") and I'd like to *group* them into the mentioned format of auther+his list of books. – Pelit Mamani Mar 03 '14 at 16:00

score 2 · Answer 3 · answered Mar 03 '14 at 16:01

2

If problem is that you don't want to explicitly write all fields (if your document have many fields and you need all of them in result), you could also try to do it with Map-Reduce:

db.books.mapReduce(
    function () { emit(this.author, this); },
    function (key, values) { return { books: values }; },
    { 
        out: { inline: 1 },
        finalize: function (key, reducedVal) { return reducedVal.books; } 
    }
)

answered Mar 03 '14 at 16:01

Ivan.Srb

1,851
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1

Nice, thank :) IMHO it's a shame such a "this" isn't supported directly in $push, for 'pipeline' syntax. But thanks very much for this interesting alternative. – Pelit Mamani Mar 04 '14 at 05:48

Mongo group and push: pushing all fields

3 Answers3

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