MongoDB Query For Fields That Vary - Wildcards? - regex

Question

I am looking for a way to get distinct "unit" values from a collection that has a structure similar to the following:
{
"_id" : ObjectId("548b1aee6e444414f00d5cf1"),
"KPI" : {
"NPV" : {
"value" : 100,
"unit" : "kUSD"
},
"NPM" : {
"value" : 100,
"unit" : "kUSD"
},
"GPM" : {
"value" : 50,
"unit" : "CAD"
}
}
}
I looked into using wildcards and regex but from what I have come across this is not supported for field matching. I would like to do something like db.collection.distinct('KPI.*.unit') but cannot determine how and it seems like performance would be poor. Does anyone have a recommendation? Thanks.

Answer 1

It's not a good practice to make the keys a part of the content of the document - don't use keys as data. If you don't change your document structure, you'll need to know what the possible subfields of KPI are. If you don't know what those could be, you will need to examine the documents manually to find them. Then you can issue a distinct for each using dot notation, e.g. db.collection.distinct("KPI.NPM.unit").
If what you're looking for instead is the distinct values of unit across all values of the parent KPI subfield, then you could take the union of all of the results of the distincts. You can also do it easily with an aggregation framework in MongoDB 2.6. For simplicity, I'll assume there's just three distinct subfields of KPI, the ones in the document above.
db.collection.aggregate([
{ "$group" : { "_id" : 0, "NPVunits" : { "$addToSet" : "$KPI.NPV.unit" }, "NPMunits" : { "$addToSet" : "$KPI.NPM.unit" }, "GPMunits" : { "$addToSet" : "$KPI.GPM.unit" } }
{ "$project" : { "distinct_units" : { "$setUnion" : ["$NPVunits", "$NPMunits", "$GPMunits"] } } }
])
You could also structure your data as dynamic attributes. The document above would be recast as something like
{
"_id" : ObjectId("548b1aee6e444414f00d5cf1"),
"KPI" : [
{ "type" : "NPV", "value" : 100, "unit" : "kUSD" },
{ "type" : "NPM", "value" : 100, "unit" : "kUSD" },
{ "type" : "GPM", "value" : 50, "unit" : "CAD" }
]
}
Querying for distinct units is easy now, whether you want it per type or over all types:
Per type (all types in one query)
db.collection.aggregate([
{ "$unwind" : "$KPI" },
{ "$group" : { "_id" : "$KPI.type", "units" : { "$addToSet" : "$KPI.unit" } } }
])
Over all types
db.collection.distinct("KPI.unit")