Aggregation on an attribute which could be null [duplicate] - mongodb

Question

This question already has answers here:
Before $unwind check if sub document is not empty
(2 answers)
Closed 3 years ago.
I am trying to aggregate attributes from two collections, one of those contains a field which may or may not be there in a document. When the attribute is not there in the document it doesn't return any document at all. So I need to create a kind of null check, that if the attribute is not there don't consider the attribute else consider it, below is my query -
db.collection(collectionName).aggregate(
[{
$match: selector
}, {
$lookup: {
from: 'status',
localField: 'candidateId',
foreignField: 'candidateId',
as: 'profile'
}
}, {
$project: {
'_id': 0,
'currentStatus': '$profile.currentStatus',
'lastContacted': '$profile.lastContacted',
'lastWorkingDay': '$profile.lastWorkingDay',
'remarks': '$profile.remarks'
}
},{
$unwind: '$lastWorkingDay'
}
In this case lastWorkingDay if not present makes the whole query return nothing. Any pointer would be helpful.

Answer 1

I believe something else is wrong with your query.
This is a bit hard to analyse without any data input, so I made up my own:
I have tried this on my local box just now, and it executes the way you'd expect it.
A projection shouldn't remove any kind of results. Here is my example:
Collection c1:
/* 1 */
{
"_id" : ObjectId("5c780eea79e5bed2bd00f85e"),
"candidateId" : "id1",
"currentStatus" : "a",
"lastContacted" : "b"
}
/* 2 */
{
"_id" : ObjectId("5c780efb79e5bed2bd00f863"),
"candidateId" : "id2",
"currentStatus" : "a",
"lastContacted" : "b",
"lastWorkingDay" : "yesterday"
}
Collection C2:
/* 1 */
{
"_id" : ObjectId("5c780f0a79e5bed2bd00f874"),
"candidateId" : "id1"
}
/* 2 */
{
"_id" : ObjectId("5c780f2879e5bed2bd00f87b"),
"candidateId" : "id2"
}
Aggregation:
db.getCollection('c2').aggregate( [
{$match: {}},
{ $lookup: {
from: "c1",
localField: "candidateId",
foreignField: "candidateId",
as : "profile"
} },
{$project: {
_id: 0,
"currentStatus" : "$profile.currentStatus",
"lastWorkingDay" : "$profile.lastWorkingDay"
} }
] )
Results:
/* 1 */
{
"currentStatus" : [
"a"
],
"lastWorkingDay" : []
}
/* 2 */
{
"currentStatus" : [
"a"
],
"lastWorkingDay" : [
"yesterday"
]
}
As you can see, the lastWorkingDay is executed correctly for both values in my aggregation.
Note that the lookup is creating an array for profiles since there could be multiple results for the lookup. You may need to unwind this if you need it in more detail.
I hope this helps.