Reference multiple fields with aggregation function

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?

Sede · Accepted Answer · 2016-12-21 12:11:51Z

2

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,
    }}
])

answered Dec 21, 2016 at 12:11

Sede

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