Join fields when not all have values - mongodb

Question

I want to modify a field through a projection stage in the aggregation pipeline, this field is combination of other fields values separated by (-)
if the field is null or empty of missing it will not be added to the cocatenated string
{$project:{
//trial-1:
finalField:{
$concat["$field1",'-','$field2','-','$field3',...]
//problem1: $concat will return null if any of it's arguments is null or missing
//problem2: if all the fields are exist with non-null values, the delimiter will exists even if the field dosen't
}
//trial-2:
finalField:{
$concat:[
{$cond:[{field1:null},'',{$concat:['$field1','-']},..]
//the problem: {field1:null} fails if the field dosen't exixt (i.e the expression gives true)
//trial-3
finalField:{
$concat:[
{$cond:[{$or:[{field1:null},{field:{$exists:true}},'',
{$concat:['$field1','-']}
]}]}
]
}
]
}
//trial-4 -> using $reduce instead of $concate (same issues)
}

Answer 1

You basically want $ifNull. It's "sort of" like $exists but for aggregation statements, where it returns a default value when the field expression returns null, meaning "not there":
{ "$project": {
"finalField": {
"$concat": [
{ "$ifNull": [ "$field1", "" ] },
"-",
{ "$ifNull": [ "$field2", "" ] },
"-",
{ "$ifNull": [ "$field3", "" ] }
]
}
}}
For example with data like:
{ "field1": "a", "field2": "b", "field3": "c" },
{ "field1": "a", "field2": "b" },
{ "field1": "a", "field3": "c" }
You get, without any error producing of course:
{ "finalField" : "a-b-c" }
{ "finalField" : "a-b-" }
{ "finalField" : "a--c" }
If you want something fancier, then you would instead dynamically work with the names, as in:
{ "$project": {
"finalField": {
"$reduce": {
"input": {
"$filter": {
"input": { "$objectToArray": "$$ROOT" },
"cond": { "$ne": [ "$$this.k", "_id" ] }
}
},
"initialValue": "",
"in": {
"$cond": {
"if": { "$eq": [ "$$value", "" ] },
"then": { "$concat": [ "$$value", "$$this.v" ] },
"else": { "$concat": [ "$$value", "-", "$$this.v" ] }
}
}
}
}
}}
Which can be aware of what fields were actually present and only attempt to join those:
{ "finalField" : "a-b-c" }
{ "finalField" : "a-b" }
{ "finalField" : "a-c" }
You can even manually specify the list of fields if you don't want the $objectToArray over the document or sub-document:
{ "$project": {
"finalField": {
"$reduce": {
"input": {
"$filter": {
"input": ["$field1", "$field2", "$field3"],
"cond": { "$ne": [ "$$this", null ] }
}
},
"initialValue": "",
"in": {
"$cond": {
"if": { "$eq": [ "$$value", "" ] },
"then": { "$concat": [ "$$value", "$$this" ] },
"else": { "$concat": [ "$$value", "-", "$$this" ] }
}
}
}
}
}}