Get a deep-nested array from a mongodb aggregate and include in result

Question

Let's say I have these two collections

book: {
    _id: 'aaa'
    name: 'Book 1',
    chapters: [
       0: {
           _id: 'chapter0',
           name: 'Chapter 1',
           pages: [
                0: {
                    _id: 'page0',
                    name: 'Page 1',
                    paragraphs: [
                        0: {
                            _id: 'paragraph0',
                            name: 'Paragraph 1',
                            bookmarks: [
                                 0: {sentence: 3, reader: 'Foo'},
                                 1: {sentence: 8, reader: 'Bar'},
                                 2: {sentence: 14, reader: 'John'}
                            ]
                        }
                    ]
                }
           ]
       }
    ]
}



book: {
    _id: 'bbb'
    name: 'Book 2',
    chapters: [
       0: {
           _id: 'chapter0',
           name: 'Chapter 1',
           pages: [
                0: {
                    _id: 'page0',
                    name: 'Page 1',
                    paragraphs: [
                        0: {
                            _id: 'paragraph0',
                            name: 'Paragraph 1',
                            bookmarks: []
                        },
                        1: {
                            _id: 'paragraph1',
                            name: 'Paragraph 2',
                            bookmarks: [
                                 0: {sentence: 2, reader: 'George'},
                                 1: {sentence: 1, reader: 'Paul'},
                                 2: {sentence: 76, reader: 'John'},
                                 3: {sentence: 54, reader: 'Ringo'}                                 
                            ]
                        }
                    ]
                }
           ]
       }
    ]
}

I want to be able to extract the array bookmarks and attach them to the book collection when getting the result. Something like this would be good:

{
    id: 'aaa'
    name: 'Book 1'
    bookmarks: [{...}, {...}, {...}] //since the first book has 3 bookmarks
},
{
    id: 'bbb'
    name: 'Book 2'
    bookmarks: [{...}, {...}, {...}, {...}] //since the second book has 4 bookmarks
},

And if there are no bookmarks, it should look like:

{
    id: 'aaa'
    name: 'Book 1'
    bookmarks: [{...}, {...}, {...}] //since the first book has 3 bookmarks
},
{
    id: 'bbb'
    name: 'Book 2'
    bookmarks: [{...}, {...}, {...}, {...}] //since the second book has 4 bookmarks
},
{
    id: 'ccc'
    name: 'Book 3'
    bookmarks: [] //third book does not have bookmarks for example
},

I've tried aggregation with this code, but it just separates each bookmark per book and pushes it into the object.

return yield Books.aggregate()
    .unwind('chapters')
    .unwind('chapters.pages')
    .unwind('chapters.pages.paragraphs')
    .unwind('chapters.pages.paragraphs.bookmarks')
    .group({
        _id: '$_id',
        books: {
            $push: {
                _id: '$_id',
                name: '$name',
                bookmarks: '$chapters.pages.paragraphs.bookmarks'
            }
        }
    }).exec()

Can someone point me to the right direction? Thanks!

sushant mehta · Accepted Answer · 2019-09-24 09:58:02Z

2

Try below aggregate pipeline:

Books.aggregate([
  {
    $unwind: "$book"
  },
  {
    $unwind: "$book.chapters"
  },
  {
    $unwind: "$book.chapters.pages"
  },
  {
    $unwind: "$book.chapters.pages.paragraphs"
  },
  {
    $unwind: {
      path: "$book.chapters.pages.paragraphs.bookmarks",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $group: {
      _id: {
        _id: "$_id",
        book: "$book.name"
      },
      bookmarks: {
        $push: "$book.chapters.pages.paragraphs.bookmarks"
      }
    }
  }
])

edited Sep 24, 2019 at 9:58

answered Sep 24, 2019 at 8:54

sushant mehta

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6 Comments

rnldpbln Over a year ago

No luck, it's returning the same data i've been getting which is the bookmarks are taking up the results one by one instead of being attached to the book object

rnldpbln Over a year ago

It works but one caveat is that it's not returning the book field if bookmarks array is empty. it should still return it but with bookmarks: []. How can i modify the code to get that result?

sushant mehta Over a year ago

add preserveNullAndEmptyArrays in last unwind then { $unwind: { path: "$book.chapters.pages.paragraphs.bookmarks", preserveNullAndEmptyArrays: true } },

rnldpbln Over a year ago

Oh btw, you should edit your answer to reflect the comment you added here. But i already accepted it

rnldpbln Over a year ago

There's one last issue though. The 'bookmarks' array is outside the _id object which I assume is used by $group. Is there any way to not use the _id object and just make it as one object that has id, name and bookmarks?

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