Mongo $concatArrays even when null - mongodb

Question

I have a large set of documents that may have two arrays or one of the two. I want to merge them in a $project.
I am currently using $concatArrays but as the documentation says it returns null when one of the arrays is null. I can figure out how to add a condition statement in there that will either return the $concatArrays or what ever array is in there.
Example
I have:
{_id: 1, array1: ['a', 'b', 'b'], array2: ['e', 'e']}
{_id: 2, array1: ['a', 'b', 'b']}
{_id: 3, array2: ['e', 'e']}
I want:
{_id: 1, combinedArray: ['a','b', 'b', 'e', 'e']}
{_id: 2, combinedArray: ['a','b', 'b']}
{_id: 3, combinedArray: ['e', 'e']}
I tried:
$project: {
combinedArray: { '$concatArrays': [ '$array1', '$array2' ] }
}
//output (unexpected result):
{_id: 1, combinedArray: ['a','b', 'b', 'e', 'e']}
{_id: 2, combinedArray: null}
{_id: 3, combinedArray: null}
I also tried:
$project: {
combinedArray: { '$setUnion': [ '$array1', '$array2' ] }
}
//output (unexpected result):
{_id: 1, combinedArray: ['a','b', 'e']}
{_id: 2, combinedArray: ['a','b']}
{_id: 3, combinedArray: ['e']}

Answer 1

As documentation for $concatArrays says
If any argument resolves to a value of null or refers to a field that
is missing, $concatArrays returns null.
So we need to be sure that we are not passing arguments which refer to a missing field or null. You can do that with $ifNull operator:
Evaluates an expression and returns the value of the expression if the
expression evaluates to a non-null value. If the expression evaluates
to a null value, including instances of undefined values or missing
fields, returns the value of the replacement expression.
So just return empty array if filed expression will not evaluate to non-null value:
db.collection.aggregate([
{$project: {
combinedArray: { '$concatArrays': [
{$ifNull: ['$array1', []]},
{$ifNull: ['$array2', []]}
] }
}
}
])

Answer 2

You can easily achieve this with the $ifNull operator:
db.arr.aggregate([
{
$project:{
combinedArray:{
$concatArrays:[
{
$ifNull:[
"$array1",
[]
]
},
{
$ifNull:[
"$array2",
[]
]
}
]
}
}
}
])
output:
{ "_id" : 1, "combinedArray" : [ "a", "b", "b", "e", "e" ] }
{ "_id" : 2, "combinedArray" : [ "a", "b", "b" ] }
{ "_id" : 3, "combinedArray" : [ "e", "e" ] }

Answer 3

I tried to do this with nested $cond, answer with $ifNull is better, but still posting my answer.
db.getCollection('test').aggregate( [{
$project: {
combinedArray: { $cond: [
{ $and: [ { $isArray: ["$array1"] }, { $isArray: ["$array2"] } ] },
{ '$concatArrays': [ '$array1', '$array2' ] },
{ $cond: [
{ $isArray: ["$array1"] },
"$array1",
"$array2"
] }
] }
}
}] )