Mongo ranking results - mongodb

Question

I have a collection like
db.books.insertMany([
{"products" : [{"name": "name1", "ids": [4, 5, 6]}], "author" : "Dante", "shelf": "a" },
{ "products" : [{"name": "name1", "ids": [4, 5]}], "author" : "Homer", "shelf": "a" },
{ "products" : [{"name": "name1", "ids": [2]}], "author" : "Dante", "shelf": "b" },
])
and I want to retrieve all documents where "shelf" is 'a'
and sort by 2 conditions:
1 - by Author
2 - documents where products.ids not contains 6 should be the first.
Could anyone help?

Answer 1

You can try this query:
First $match the shelf value with "a".
Then create an auxiliar value where will be true if 6 not exists into products.ids, otherwise false.
Then $sort by values you want.
And use $project to remove the auxiliar value.
db.collection.aggregate([
{
"$match": {"shelf": "a"}
},
{
"$set": {
"rank": {
"$eq": [
{
"$filter": {
"input": "$products",
"cond": {"$in": [6,"$$this.ids"]}
}
},[]
]
}
}
},
{
"$sort": {
"rank": -1,
"author": 1
}
},
{
"$project": {"rank": 0}
}
])
Example here

Answer 2

Here is a variation that sorts more granularly on author+"not containing 6".
db.foo.aggregate([
{$match: {shelf:'a'}}
,{$unwind: '$products'}
,{$addFields: {sortMarker: {$cond: [
{$in: [6, '$products.ids']},
"Z", // THEN make sortMarker at the end
"A" // ELSE make sortMarker at the start
]}
}}
,{$sort: {'author':1, 'sortMarker':1}}
]);
which given this input set:
{"products" : [
{"name": "name3", "ids": [6, 7]},
{"name": "name2", "ids": [4, 5]}
],
"author" : "Homer",
"shelf": "a" },
{"products" : [
{"name": "name1", "ids": [4, 5, 6]},
{"name": "name4", "ids": [9]},
{"name": "name7", "ids": [9,6]},
{"name": "name7", "ids": [10]}
],
"author" : "Dante",
"shelf": "a"},
{ "products" : [
{"name": "name1", "ids": [2]}
], "author" : "Dante",
"shelf": "b"}
yields this result:
{
"_id" : 1,
"products" : {
"name" : "name4",
"ids" : [
9
]
},
"author" : "Dante",
"shelf" : "a",
"sortMarker" : "A"
}
{
"_id" : 1,
"products" : {
"name" : "name7",
"ids" : [
10
]
},
"author" : "Dante",
"shelf" : "a",
"sortMarker" : "A"
}
{
"_id" : 1,
"products" : {
"name" : "name1",
"ids" : [
4,
5,
6
]
},
"author" : "Dante",
"shelf" : "a",
"sortMarker" : "Z"
}
{
"_id" : 1,
"products" : {
"name" : "name7",
"ids" : [
9,
6
]
},
"author" : "Dante",
"shelf" : "a",
"sortMarker" : "Z"
}
{
"_id" : 0,
"products" : {
"name" : "name2",
"ids" : [
4,
5
]
},
"author" : "Homer",
"shelf" : "a",
"sortMarker" : "A"
}
{
"_id" : 0,
"products" : {
"name" : "name3",
"ids" : [
6,
7
]
},
"author" : "Homer",
"shelf" : "a",
"sortMarker" : "Z"
}
Optionally, this stage can be added after the $sort:
{$group: {_id: '$author', products: {$push: '$products'}}}
And this will bring the sorted "not containing 6 then containing 6" items together again as an array packaged by author; the $push retains the order. Note we need only need author in _id because the match was for one shelf. If more than one shelf is in the match, then we would need:
{$group: {_id: {author:'$author',shelf:'$shelf'}, products: {$push: '$products'}}}