MongoDB: adding fields based on partial match query - expression vs query - mongodb

Question

So I have one collection that I'd like to query/aggegate. The query is made up of several parts that are OR'ed together. For every part of the query, I have a specific set of fields that need to be shown.
So my hope was to do this with an aggregate, that will $match the queries OR'ed together all at once, and then use $project with $cond to see what fields are needed. The problem here is that $cond uses expressions, while the $match uses queries. Which is a problem since some query features are not available as an expression. So a simple conversion is not an option.
So I need another solution..
- I could just make an aggregate per separate query, because there I know what fields to match, and them merger the results together. But this will not work if I use pagination in the queries (limit/skip etc).
- find some other way to tag every document so I can (afterwards) remove any fields not needed. It might not be super efficient, but would work. No clue yet how to do that
- figure out a way to make queries that are only made of expressions. For my purpose that might be good enough, and it would mean a rewrite of the query parser. It could work, but is not ideal.

Answer 1

So This is the next incarnation right here. It will deduplicate and merge records and finally transform it back again to something resembling a normal query result:
db.getCollection('somecollection').aggregate(
[
{
"$facet": {
"f1": [
{
"$match": {
<some query 1>
},
{
"$project: {<some fixed field projection>}
}
],
"f2": [
{
"$match": {
<some query 1>
}
},
{
"$project: {<some fixed field projection>}
}
]
}
},
{
$project: {
"rt": { $concatArrays: [ "$f1", "$f2"] }
}
},
{ $unwind: { path: "$rt"} },
{ $replaceRoot: {newRoot:"$rt"}},
{ $group: {_id: "$_id", items: {$push: {item:"$$ROOT"} } }},
{
$project: {
"rt": { $mergeObjects: "$items" }
}
},
{ $replaceRoot: {newRoot:"$rt.item"}},
]
);
There might still be some optimisation to be, so any comments are welcome

Answer 2

I found an extra option using $facet. This way, I can make a facet for every group opf fields/subqueries. This seems to work fine, except that the result is a single document with a bunch of arrays. not yet sure how to convert that back to multiple documents.

Answer 3

okay, so now I have it figured out. I'm not sure yet about all of the intricacies of this solution, but it seems to work in general. Here an example:
db.getCollection('somecollection').aggregate(
[
{
"$facet": {
"f1": [
{
"$match": {
<some query 1>
},
{
"$project: {<some fixed field projection>
}
],
"f2": [
{
"$match": {
<some query 1>
}
},
{
"$project: {<some fixed field projection>
}
]
}
},
{
$project: {
"rt": { $concatArrays: [ "$f1", "$f2"] }
}
},
{ $unwind: { path: "$rt"} },
{ $replaceRoot: {newRoot:"$rt"}}
]
);