How to include external array inside MongoDB aggregation? - mongodb

Question

In aggregation project array element from index x to y where x is defined inside collection and y is an array element corresponding to the index of the array element which is matched. Please see below example then it will become quite easier to understand what i am trying to say.
coupons collection
{"coupon_id": "coupon01", "codes": ["FLAT30", "FLAT50", "FLAT70", "FLAT90"], "curr_index": 0}
For example see below example code here i am trying to get coupon codes starting from curr_index to (curr_index + n) where n is the number in coupon_ctrs corresponding to the index of coupons_ids example- for id "584559bd1f65d363bd5d25fd" n is 1, for id "58455a5c1f65d363bd5d2600" n is 2 and for the id "584878eb0005202c64b0cc5d" n is 3.
coupons_ctrs = [1, 2, 3];
coupons_ids = ["584559bd1f65d363bd5d25fd", "58455a5c1f65d363bd5d2600", "584878eb0005202c64b0cc5d"];
int n = 2;
couponmodel.aggregate(
{ $match : { '_id': { $in : coupons_ids }} },
{ $project: {_id:0, codes : /* How to use slice here so that codes array will be returned between cur_index and (curr_index + coupons_ctr corresponding to the index coupon id is found, example- for _id "584559bd1f65d363bd5d25fd" it should be 1 and so on) */} },
function(err, docs) {
if (err) {
} else {
}
});
UPDATE
As suggested by Styvane i could use $zip and it would work perfect but as i am using mongoDB 3.2.11 so i can't use it, so what can be the solution for using functionality of $zip in mongodb 3.2.11 ??
Can anyone please tell me how can i include this coupon_ctrs array and use it inside aggregation pipeline.

Answer 1

You need to apply a $slice expression to each element in the "coupons_ctrs" array using the $map operator which means that we use the literal "coupons_ctrs" array as "input" to $map.
let coupons_ids = [
"584559bd1f65d363bd5d25fd",
"58455a5c1f65d363bd5d2600",
"584878eb0005202c64b0cc5d"
];
let coupons_ctrs = [1, 2, 3];
db.couponmodel.aggregate(
[
{ "$match" : { "_id": { "$in" : coupons_ids } } },
{ "$project": {
"codes": {
"$map": {
"input": coupons_ctrs,
"as": "n",
"in": {
"$slice": [
"$codes",
"$curr_index",
{ "$add": [ "$curr_index", "$$n" ] }
]
}
}
}
}}
]
)
Which yields:
{
"codes" : [
[ "FLAT30" ],
[ "FLAT30", "FLAT50" ],
[ "FLAT30", "FLAT50", "FLAT70" ]
]
}
In MongoDB 3.4 we can use the $zip operator to do this:
db.couponmodel.aggregate(
[
{ "$project": {
"codes": {
"$map": {
"input": {
"$zip": {
"inputs": [ coupons_ids, coupons_ctrs ]
}
},
"as": "item",
"in": {
"coupon_id": { "$arrayElemAt": [ "$$item", 0 ] },
"value": {
"$slice": [
"$codes",
"$curr_index",
{ "$add": [
"$curr_index",
{ "$arrayElemAt": [ "$$item", 1 ] }
] }
]
}
}
}
}
}}
]
)
which return something like this:
{
"codes" : [
{
"coupon_id" : "584559bd1f65d363bd5d25fd",
"value" : [ "FLAT30" ]
},
{
"coupon_id" : "58455a5c1f65d363bd5d2600",
"value" : [ "FLAT30", "FLAT50" ],
},
{
"coupon_id" : "584878eb0005202c64b0cc5d",
"value" : [ "FLAT30", "FLAT50", "FLAT70" ]
}
]
}