How to query nested array of objects and group & count by one of its properties? Tips?

[steevej (Steeve Juneau)]

I am pretty sure you may apply the same solution as

Project a counter of occurrences from document array

This is NOT a good way - you should never use $unwind and then $group when you just need to transform a single document! If you have an array flags: [ 'a', 'b', 'x', 'y', ... ] the way to get number of unique entries is like this: {$addFields:{ flags: {$map: { input: {$setUnion:"$flags"}, /* unique elements of flags array */ as: "f", in: { name: "$$f", count: {$size: {$filter:{input:"$flags", cond:{$eq:["$$this", "$$f"]}}}} …