mongodb: check condition in aggregation - mongodb

Question

I am using mongodb 3.6. I have many document in my collection.inside the document i do not have any Domain field. I create Domain for some document.
Now I want to use aggregate for filtering this collection. that is mean, I want those documents that have not Domain as a field.
db.Events.aggregate([
{$project : {
Domain : {$filter: {
input: "$Domain",
cond:{if: {Domain : {$exists: false}}, then: {"$BusinessCode": 1} }}}
}
}
],{
allowDiskUse: true
})
when I execute this script I got error:
Assert: command failed: {
"ok" : 0,
"errmsg" : "Unrecognized expression '$exists'",
"code" : 168,
"codeName" : "InvalidPipelineOperator"
} : aggregate failed
seems $exists is not supported into $filter expression.
How could I do that?
Another question is: Can I use 2 $project like this:
db.Events.aggregate([
{$project : {
Domain : {$filter: {
input: "$Domain",
cond:{if: {Domain : {$exists: false}}, then: {"$BusinessCode": 1} }}}
}
},
{
$match : {BusinessCode: /(([1-2]?[0-9])-([0-9]*)-([0-9]*)-([0-9]*)-([0-9]*)-([0-9]*)-([0-9]*))/}
},
{
$project : {BusinessCode : {$arrayElemAt:[{$split : ["$BusinessCode", "-"]},0]}}
},
{
$addFields: {"Domain": "$BusinessCode"}
},
],{
allowDiskUse: true
})
I want to check, does specific field is there into document. if does not exist, BusinessCode projected and other stuff..
***************************Edit****************
this is my sample of documents:
"DeviceId" : "xxxxxxx",
"UserId" : UUID(""),
"UserFullName" : "test-user",
"SystemId" : "com.messaging",
"SystemTitle" : "message",
"EventId" : "messaging.message",
"EventTitle" : "test",
"EventData" : [],
"BusinessCode" : "1-2-4-4-5-6-9",...
After execute this script, I expect "Domain" append to my document like this:
"EventTitle" : "test",
"EventData" : [],
"BusinessCode" : "1-2-4-4-5-6-9"
"Domain": "1" // 1 is first number of BusinessCode that splitted
but if Domain was exist script goes to next document and check again.

Answer 1

So you're looking for a something like COALESCE in SQL and it is called $ifNull in MongoDB. For instance:
db.Events.save({Domain: "4"})
db.Events.save({BusinessCode: "1-2-4-4-5-6-9"})
db.Events.aggregate([
{
$project: {
Domain: {
$ifNull: [ "$Domain", { $arrayElemAt: [ { $split : ["$BusinessCode", "-"] },0] } ] }
}
}
])