Mongodb :- merging object ifNull then return data if present - mongodb

Question

Extension of this Question, accepted answer is working but there are some things which i am not understanding.
Everything is working but there is one more thing in this is if array1 i.e shiftListData does not have any data of any date for example lets take 25th Jan 2023 but array2 i.e attendances have data of 25th Jan 2023 then it will not merge both object and return nothing i.e it will skip that date even if data is present on array2

Answer 1

From my interpretation, the question is more concerned about "patching" the info from attendances into shiftListData. While your concern is correct and understandable, I would say the case may not be that meaningful since it will not create meaningful, merged shiftListData in this case.
However, the above is just my personal interpretation. We can still solve your question by applying the same idea from the original solution. We just need to revert the merging order to create another merged result. Use $setUnion to join them together to create the final merged list.
db.collection.aggregate([
{
$set: {
shiftListData1: {
$map: {
input: "$shiftListData",
as: "shift",
in: {
$mergeObjects: [
"$$shift",
{
$ifNull: [
{
$first: {
$filter: {
input: "$attendances",
cond: {
$eq: [
"$$this.Date",
"$$shift.date"
]
}
}
}
},
{}
]
}
]
}
}
},
shiftListData2: {
$map: {
input: "$attendances",
as: "a",
in: {
$mergeObjects: [
{
$ifNull: [
{
$first: {
$filter: {
input: "$shiftListData",
cond: {
$eq: [
"$$this.date",
"$$a.Date"
]
}
}
}
},
{}
]
},
"$$a"
]
}
}
},
attendances: "$$REMOVE"
}
},
{
"$project": {
shiftListData: {
$setUnion: [
"$shiftListData1",
"$shiftListData2"
]
}
}
}
])
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