Query inner value in mongoDB - mongodb

Question

all
I'm trying to do a join in MongoDB but also, I need to check for conditions and to do a sum on inner values of what comes back from the join.
I will explain.
Currently I have this simple join query which looks like this:
db.Sets.aggregate([
{
$lookup:
{
from: "ExecutionTasks",
localField: "identifier",
foreignField: "setIdentifier",
as: "execTask"
}
}
])
It returns the following results:
/* 1 */
{
"_id" : 1,
"name" : "Demo Set",
"identifier" : "demo-set",
"description" : "Demo Set",
"creator" : {
"id" : 1,
"name" : "admin"
},
"createdDate" : ISODate("2017-03-24T20:09:55.120Z"),
"updatedDate" : ISODate("2017-03-24T20:09:55.120Z"),
"execTask" : [
{
"_id" : 1,
"isActive" : 1,
"type" : "count",
"threshold" : {
"default" : "0",
"deviations" : []
},
"name" : "amishay",
"setIdentifier" : "demo-set",
"description" : "a",
"query" : {
"source" : 1,
"text" : "select * from t"
},
"creator" : {
"id" : 1,
"name" : "admin"
},
"createdDate" : ISODate("2017-03-27T20:03:22.275Z"),
"updatedDate" : ISODate("2017-03-27T20:03:22.275Z")
},
{
"_id" : 2,
"isActive" : 0,
"type" : "count",
"threshold" : {
"default" : "0",
"deviations" : []
},
"name" : "amishay2",
"setIdentifier" : "demo-set",
"description" : "test",
"query" : {
"source" : 1,
"text" : "select * from t"
},
"creator" : {
"id" : 1,
"name" : "admin"
},
"createdDate" : ISODate("2017-03-27T20:03:57.248Z"),
"updatedDate" : ISODate("2017-03-27T20:03:57.248Z")
}
]
}
What I would like to do is to return only the length of the array (execTask) and also only those with the attribute isActive which equals to 1.
So basically I want to get something like:
{
"_id" : 1,
"name" : "Demo Set",
"identifier" : "demo-set",
"description" : "Demo Set",
"creator" : {
"id" : 1,
"name" : "admin"
},
"createdDate" : ISODate("2017-03-24T20:09:55.120Z"),
"updatedDate" : ISODate("2017-03-24T20:09:55.120Z"),
"execTask" : 1
}
I checked online numerous questions but I only saw examples which query the collection attribute and not the joined collection attribute.
Thanks!

Answer 1

You can add $addFields stage after $lookup. The below stage will $filter and calculate the $size for query criteria.
$filter operator is to used to filter the execTask array contents in-place on the mentioned criteria.
Expressions $ and $$ to reference the fields / aggregation operators / aggregation stages and inner variables respectively.
$size operator to calculate the length of filtered array.
$addFields overwrites the existing field execTask to replace its value with the calculated size.
{
$addFields: {
"execTask": {
$size: {
$filter: {
input: "$execTask",
as: "result",
cond: {
$eq: ["$$result.isActive", 1]
}
}
}
}
}
}