MongoDB query or aggregation to skip sub-documents - mongodb

Question

I'd like to create a query or aggregation where the returned documents do not include sub-documents. I do not know that a given field will be a sub-document ahead of time (or I would just use the projection to skip them). So for example, if I have a document like this:
{
_id: 1,
field1: "a",
field2: "b",
field3: {
subfield1: "c",
subfield2: "d"
}
}
When my query returns this document, it either skips field3, or replaces field3's value with something else (e.g. a string = "field_is_an_object").
As I said, I don't know ahead of time which fields will be sub-documents (or "object" types). The $redact operator was the closest I could find, but I couldn't figure out a syntax to get it to work.

Answer 1

There are at least two ways you can achieve what you want:
The first one is pretty concise and requires just one aggregation stage which, however, is a little bit more complex and harder to understand:
db.collection.aggregate({
$replaceRoot: { // create a new top level document
"newRoot": { // ...which shall be
$arrayToObject: { // ...created from an array
$filter: { // ...that again should contain only those elements
input: { // ...from our input array
$objectToArray: "$$ROOT" // ...which is our respective top level document transformed into an array of key-value pairs
},
cond: { // ...where
$ne: [ { $type: "$$this.v" }, "object" ] // ...the "v" (as in "value" field is not an object)
}
}
}
}
}
})
The second one I can think of is way more verbose but pretty easy to understand by adding the stages step-by-step (as always with the aggregation framework).
db.collection.aggregate({
$project: {
"tmp": { // we create a temporary field
$objectToArray: "$$ROOT" // that contains our respective root document represented as an array of key-value pairs
}
}
}, {
$unwind: "$tmp" // flatten the temporary array into multiple documents
}, {
$match: {
"tmp.v": { $not: { $type: "object" } } // filter all documents out that we do not want in our result
}
}, {
$group: { // group all documents together again
"_id": "$_id", // into one bucket per original document ("_id")
"tmp": {
$push: "$tmp" // and create an array with all the key-value pairs that have survived our $match stage
}
}
}, {
$replaceRoot: { // create a new top level document...
"newRoot": {
$arrayToObject: "$tmp" // ...out of the data we have left in our array
}
}
})