Using cond to specify _id fields for group in mongodb aggregation - mongodb

Question

new to Mongo. Trying to group across different sub fields of a document based on a condition. The condition is a regex on a field value. Looks like -
db.collection.aggregate([{
{
"$group": {
"$cond": [{
"upper.leaf": {
$not: {
$regex: /flower/
}
}
},
{
"_id": {
"leaf": "$upper.leaf",
"stem": "$upper.stem"
}
},
{
"_id": {
"stem": "$upper.stem",
"petal": "$upper.petal"
}
}
]
}
}])
Using api v4.0: cond in the docs shows - { $cond: [ <boolean-expression>, <true-case>, <false-case> ] }
The error I get with the above code is - "Syntax error: dotted field name 'upper.leaf' can not used in a sub object."
Reading up on that I tried $let to re-assign the dotted field name. But started to hit various syntax errors with no obvious issue in the query.
Also tried using $project to rename the fields, but got - Field names may not start with '$'
Thoughts on the best approach here? I can always address this at the application level and split my query into two but it's attractive potentially to solve it natively in mongo.

Answer 1

$group syntax is wrong
{
$group:
{
_id: <expression>, // Group By Expression
<field1>: { <accumulator1> : <expression1> },
...
}
}
You tried to do
{
$group:
<expression>
}
And even if your expression resulted in the same code, its invalid syntax for $group (check from the documentation where you are allowed to use expressions)
One other problem is that you use the query operator for regex, and not the aggregate regex operators (you can't do that, if you aggregate you can use only aggregate operators, only $match is the exception that you can use both if you add $expr)
You need this i think
[{
"$group" : {
"_id" : {
"$cond" : [ {
"$not" : [ {
"$regexMatch" : {
"input" : "$upper.leaf",
"regex" : "/flower/"}}]},
{"leaf" : "$upper.leaf","stem" : "$upper.stem"},
{"stem" : "$upper.stem","petal" : "$upper.petal"}]
}
}}]
Its similar code, but expression gets as value of the "_id" and $regexMatch
is used that is aggregate operator.
I didnt tested the code.