Total count will always be a one-document result so you need $facet to run mutliple aggregation pipelines and then merge results. Let’s say your regular pipeline contains simple $project
and you want to merge it’s results with $count
. You can run below aggregation:
db.col.aggregate([
{
$facet: {
totalCount: [
{ $count: "value" }
],
pipelineResults: [
{
$project: { _id: 1 } // your regular aggregation pipeline here
}
]
}
},
{
$unwind: "$pipelineResults"
},
{
$unwind: "$totalCount"
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [ "$pipelineResults", { totalCount: "$totalCount.value" } ]
}
}
}
])
After $facet
stage you’ll get single document like this
{
"totalCount" : [
{
"value" : 3
}
],
"pipelineResults" : [
{
"_id" : ObjectId("5b313241120e4bc08ce87e46")
},
//....
]
}
Then you have to use $unwind to transform arrays into multiple documents and $replaceRoot with $mergeObjects to promote regular pipeline results into root level.