Mongoose - Find object with any key and specific subkey - mongodb

Question

Let's say I have a Mongo database that contains objects such as :
[
{
"test": {
"123123": {
"someField": null
}
}
},
{
"test": {
"323143": {
"someField": "lalala"
},
"121434": {
"someField": null
}
}
},
{
"test": {
"4238023": {
"someField": "afafa"
}
}
},
]
As you can see, the keys right under "test" can vary.
I want to find all documents that have at least one someField that is not null.
Something like find : "test.*.someField": { $ne: null } ( * represents any value here)
How can i do this in mongoose ? I'm thinking an aggregation pipeline will be needed here but not exactly sure how.
Constraints :
I don't have much control over the db schema in this scenario.
Ideally i don't want to have to do this logic in nodeJS, I would like to query directly via the db.

Answer 1

The trickiest part here is that you cannot search keys that match a pattern. Luckily there is a workaround. Yes, you do need an aggregation pipeline.
Let's look at an individual document:
{
"test": {
"4238023": {
"someField": "afafa"
}
}
}
We need to query someField, but to get to it, we need to somehow circumvent 4238023 because it varies with each document. What if we could break that test object down and look at it presented like so:
{
"k": "4238023",
"v": {
"someField": "afafa"
}
}
Suddenly, it get a heck of a lot easier to query it. Well, mongodb aggreation offers a function called $objectToArray which does exactly that.
So what we are going to do is:
Convert the test object into an array for each document.
Match only documents where AT LEAST ONE v.someField is not null.
Put it back together to look as your original documents, minus the ones that do not match the null criterion.
So, here is the pipeline you need:
db.collection.aggregate([
{
"$project": {
"arr": {
"$objectToArray": "$$ROOT.test"
}
}
},
{
"$match": {
arr: {
$elemMatch: {
"v.someField": {
$ne: null
}
}
}
}
},
{
"$project": {
"_id": 1,
"test": {
$arrayToObject: "$arr"
}
}
}
])
Playground: https://mongoplayground.net/p/b_VNuOLgUb2
Note that in mongoose you will run this aggregation the same way you would do it in a terminal... well plus the .then.
YourCollection.aggregate([
...
...
])
.then(result => console.log(result))