how to count/sum array on mongodb? - mongodb

Question

I have a collection where every documents has an array named "contacts".
Sample Docs :
{
"_id" : ObjectId("5660c2dfdfdfcba2d47baa2d9"),
"name" : john,
"contacts" : [
{
"name" : "ari",
"phone" : "12341234"
},
{
"name" : "dea",
"phone" : "34234234"
}
]
},
{
"_id" : ObjectId("5660c2dfdfdfcba2d47baa2d9"),
"name" : joni,
"contacts" : [
{
"name" : "budi",
"phone" : "13341234"
},
{
"name" : "ade",
"phone" : "3242343"
},
{
"name" : "are",
"phone" : "64545345"
}
]
}
I want to know get total count of contacts for all docs.
From sample docs output should be 5 contacts
Thank you for helping me.

Answer 1

You can try below query :
1) If contacts array exists in every doc :
db.collection.aggregate([
/** project only needed field contacts with size of array in each doc */
{
$project: {
_id: 0,
contacts: {
$size: "$contacts"
}
}
},
/** group on empty(without any filter) & sum contacts field */
{
$group: {
_id: "",
TotalContacts: {
$sum: "$contacts"
}
}
},
/** Optional projection */
{
$project: {
_id: 0
}
}
])
2) If contacts array field might not exist in every doc, if it exists & not an array then an additional type check has to be done in below $cond :
db.collection.aggregate([
/** group on empty(without any filter) & sum contacts field's size if contacts exists else sum 0, You've can have optional projection as first stage like above query */
{
$group: {
_id: "",
TotalContacts: {
$sum: {
$cond: [
{
"$ifNull": [
"$contacts",
false
]
},
{
$size: "$contacts"
},
0
]
}
}
}
},
/** Optional projection */
{
$project: {
_id: 0
}
}
])
Test : MongoDB-Playground