Reference multiple fields with aggregation function - mongodb

Question

Let's say I have some mongo DB query which returns following two documents. (I am using aggregation & projection which returns me this result set).
{
"name" : {
"value" : "ANDERSON"
},
"ID" : {
"value" : "2356"
},
}
{
"employeename" : {
"value" : "DAVID"
},
"ID" : {
"value" : "2356"
},
}
My DB is schema less & I am storing attributes and there values. There are multiple attributes which represents the same information. For e.g. here "name" & "employeename" represents the same thing. I want the final output in some common attribute (say "Employee Name"). This common attribute can have value either from "name" or "employeename".
I think this problem can be solved by adding one more pipe in with the aggregation. I tried $or (it returns true/false not the value)
db.getCollection('mycollection').aggregate([
{ "$project" : {
"name" : 1,
"ID" : 1, "employeename" : 1
}},
{ "$project":{
"Employee Name": {$or : ["$name", "$employeename"]}
}}
])
Final Output should be
{
" Employee Name" : {
"value" : "ANDERSON"
},
"ID" : {
"value" : "2356"
},
}
{
" Employee Name" : {
"value" : "DAVID"
},
"ID" : {
"value" : "2356"
},
}
Can somebody tell me how to write this mongo DB command?

Answer 1

What you want is the $ifNull operator, you can also shorten your pipeline to one $project stage.
db.getCollection('mycollection').aggregate([
{ "$project" : {
"EmployeeName" : { "$ifNull": [ "$name", "$employeename" ] },
"ID" : 1,
}}
])