how to conditionally concatenate fields in mongodb aggregation

Question

I want to build a string from concatenating some fields, but I want to check the value of each field before I decide to concatenate it.

//syntax:
{$projection:{fieldName:{$concat:["$field1","-","$field2","$field3"]}}}

what I want?

fieldName=($field1!=null?"$field1-":"")+"$field2"+($field3=="ok"?"approved":"pending")

Steven Spungin · Accepted Answer · 2019-08-06 14:13:51Z

8

For the first case (null), you would use the $ifNull operator. This will return a default value if the field is null or missing.

For the second case, use the $cond operator.

fieldName:{
 $concat:
 [
  {$ifNull:["$field1", ""]},
  "-",
  "$field2", 
  {$cond:[{$eq:['$field3', 'ok']}, "approved", "pending"] }
 ]
}

answered Aug 6, 2019 at 14:13

Steven Spungin

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