Temporary Tables (CTE) in MongoDB - mysql

Question

I know that common table expressions (CTE) a.k.a. "temporary named result sets" can be used in SQL to generate a temporary table, but can this be done in MongoDB? I want a document, but it's only for temporary use in my query.
Can you create a temporary table in MongoDB without creating a new collection?
For example, if I were to try to recreate the code below in Mongo...
Example CTE Table in SQL:
n
f1
f2
1
20
12
2
40
0.632
3
60
0.647
WITH RECURSIVE example (n, f1, f2) AS
( SELECT 1, 20, 12
UNION ALL SELECT
n + 1,
n * 20,
least(6*n, $globalVar * 100),
FROM example WHERE n < 3
) SELECT * FROM example

Answer 1

It seems that there is no general equivalent for CTE in MongoDB. However, for OP's example, it is possible to wrangle the output of $range to produce a similar effect.
// whichever collection doesn't matter; as long as it has 1 document then it should be fine
db.collection.aggregate([
{
// jsut take 1 document
"$limit": 1
},
{
// use $range to generate iterator [1, 2, 3]
"$addFields": {
"rg": {
"$range": [
1,
4
]
},
globalVar: 0.001
}
},
{
// do the mapping according to logic
"$addFields": {
"cte": {
"$map": {
"input": "$rg",
"as": "n",
"in": {
n: "$$n",
f1: {
"$multiply": [
"$$n",
20
]
},
f2: {
"$cond": {
"if": {
$lt: [
{
"$multiply": [
"$$n",
6
]
},
{
"$multiply": [
"$globalVar",
100
]
}
]
},
"then": {
"$multiply": [
"$$n",
6
]
},
"else": {
"$multiply": [
"$globalVar",
100
]
}
}
}
}
}
}
}
},
{
// wrangle back to expected form
"$unwind": "$cte"
},
{
"$replaceRoot": {
"newRoot": "$cte"
}
}
])
Here is the Mongo playground for your reference.