MongoDB aggregation framework: group query - mongodb

Question

i'm looking to get a specific query using the aggregation framework of mongoDB. I think i need the $group and $addToSet operators but i'm confused about the right query to use.
This is the article collection:
/* 0 */
{
"_id" : 4,
"author" : "Kevin Vanhove",
"book" : {
"order" : 500,
"title" : "HTML",
"url" : "html"
},
"chapter" : {
"img" : "navChapter-logo",
"order" : 500,
"title" : "W3C",
"url" : "w3c"
},
"featured" : 0,
"heading" : [
{
"title" : "title1",
"_id" : ObjectId("53130fb8b9b9f573a877401d")
},
{
"title" : "title2",
"_id" : ObjectId("53130fb8b9b9f573a877401c")
}
],
"intro" : "Some intro text",
"title" : "Internet with and without the W3C",
"url" : "internet-with-and-without-the-W3C"
}
/* 1 */
{
"_id" : 1,
"author" : "Kevin Vanhove",
"book" : {
"order" : 500,
"title" : "Javascript",
"url" : "javascript"
},
"chapter" : {
"img" : "navChapter-logo",
"order" : 500,
"title" : "Functions",
"url" : "functions"
},
"featured" : 1,
"heading" : [
{
"title" : "Parts of a function",
"_id" : ObjectId("53130e0cd8517b65a614c1ab")
},
{
"title" : "Something else",
"_id" : ObjectId("53130e0cd8517b65a614c1aa")
}
],
"intro" : "Some intro text",
"title" : "A visual illustration of the JS function",
"url" : "a-visual-illustration-of-the-javascript-function"
}
/* 2 */
{
"_id" : 2,
"author" : "Kevin Vanhove",
"book" : {
"order" : 500,
"title" : "Javascript",
"url" : "javascript"
},
"chapter" : {
"img" : "navChapter-logo",
"order" : 300,
"title" : "Variables",
"url" : "variables"
},
"featured" : 1,
"heading" : [
{
"title" : "Global vs local",
"_id" : ObjectId("53130ea8a9dc9c28a77ea28a")
},
{
"title" : "Variable hoisting",
"_id" : ObjectId("53130ea8a9dc9c28a77ea289")
},
{
"title" : "The scope chain",
"_id" : ObjectId("53130ea8a9dc9c28a77ea288")
}
],
"intro" : "Some intro text",
"title" : "How variable scope works in javascript",
"url" : "how-variable-scope-works-in-javascript"
}
/* 3 */
{
"__v" : 0,
"_id" : 3,
"author" : "Kevin Vanhove",
"book" : {
"order" : 500,
"title" : "Javascript",
"url" : "javascript"
},
"chapter" : {
"img" : "navChapter-logo",
"order" : 600,
"title" : "Functions",
"url" : "functions"
},
"featured" : 0,
"heading" : [
{
"title" : "title1",
"_id" : ObjectId("53130f60f0de2506a81e2d62")
},
{
"title" : "title2",
"_id" : ObjectId("53130f60f0de2506a81e2d61")
}
],
"intro" : "Some intro text",
"title" : "Javascript closures, in depth",
"url" : "Javascript-closure-in-depth"
}
I need to have all the 'unique' books and their 'unique' chapters (no duplicates), so i use this query:
/*
db.articles.aggregate({$group : {_id : "$book.title", chapters:{$addToSet:"$chapter.title"}}})
*/
This gives me this result:
/* 0 */
{
"result" : [
{
"_id" : "Javascript",
"chapters" : [
"Variables",
"Functions"
]
},
{
"_id" : "HTML",
"chapters" : [
"W3C"
]
}
],
"ok" : 1
}
That is almost what i want but not completely. What actually need is this:
/* 0 */
{
"result" : [
{
"_id" : "Javascript",
"chapters" : [
{
"img" : "navChapter-logo",
"order" : 600,
"title" : "Functions",
"url" : "functions"
},
{
"img" : "navChapter-logo",
"order" : 300,
"title" : "Variables",
"url" : "variables"
}
]
},
{
"_id" : "HTML",
"chapters" : [
{
"img" : "navChapter-logo",
"order" : 500,
"title" : "W3C",
"url" : "w3c"
}
]
}
],
"ok" : 1
}
So i need all the unique books with their unique chapters, but i also want the extra fields added like "order" and "url". The query i'm using at the moment only gives me the chapter titles.
Update:
I also tried: $addToSet:"$chapter" instead of $addToSet:"$chapter.title"...
But now i get duplicates on the chapter.title field. I should get only 2 distinct chapters in book 'javascript', and now i get 3 chapters (1 duplicate)

Answer 1

You can use $group to aggregate by book and chapter.
db.articles.aggregate(
{$group : {_id : {t:"$book.title",c:"$chapter.title"},
img:{$first:"$chapter.img"},
url:{$first:"$chapter.url"},
order:{$sum:"$chapter.order"}
}})
In case order was important I kept it and added them across the "duplicate" chapters for same book.