Membership testing in conditional operator in mongo - mongodb

Question

How can I use $in (like) operator inside $cond? For example, inside a "project" stage I want to create a new field called "category" that is conditional on some other fields (for grouping later): it's value would be catA if "test_value" is in the list [1,9] else "catB". For small list I can use set of $eq inside "$or".
"category": {
"$cond": {
"if" :{
"$or" : [
{ "$eq" : ["$test_value", 1]},
{"$eq" : ["$test_value", 9] }
]
},
"then": {
"$literal" : "catA"
},
"else": {
"$literal" : "catB"
}
}
Is there any way to use something like:
"if" :{
"$in" : {"$test_value", [1,9]}
},
"then": {
"$literal" : "catA"
},
"else": {
"$literal" : "catB"
}
}
I am using mongoengine, and above syntax is not allowed.

Answer 1

You can do intersection of the field and the list of expected values for that field using $setIntersection and check the length of the result. If the length is bigger that 0 then the field has value from the list of expected values.
Try the following query:
db.collection.aggregate([
{
$project: {
category: {
$let: {
vars: {
resultSize: {
$size: { $setIntersection: [["$test_value"], [1, 9]] }
}
},
in: {
"$cond": {
"if": { "$gt" : ["$$resultSize", 0] },
"then": { "$literal" : "catA" },
"else": { "$literal" : "catB" }
}
}
}
}
}
}
])