MongoDB查询嵌入式文档时如何扁平化投影指定字段？

Got it, let's solve this. You want to pull that nested instrumentRanges.flute.minPitch up to the top level as minPitch in your find() results, no extra map() or client-side processing needed. Here's how to do it:

Working Query (MongoDB 4.4+) #

Starting in MongoDB 4.4, the projection parameter in find() supports aggregation-style field expressions. You can directly map the nested field to a top-level key like this:

db.scoreFacts.find(
  {"instrumentRanges.flute.minPitch": {$gte: 0, $lte:56}},
  {
    "_id": 1,
    "minPitch": "$instrumentRanges.flute.minPitch"
  }
)

This will return exactly what you're expecting:

{ "_id" : "Bach_Brandenburg5_Mov1.xml", "minPitch" : 50 }

Why Your Original Query Didn't Work #

Your initial projection {"instrumentRanges.flute.minPitch": 1} tells MongoDB to include that specific nested field, but it preserves the original hierarchy. MongoDB doesn't automatically flatten nested fields unless you explicitly map them to top-level keys.

For Older MongoDB Versions (Pre-4.4) #

If you're stuck on a version before 4.4, you'll need to use the aggregation pipeline (which is still a server-side operation, no client-side map() required). It's not strictly find(), but it's the closest equivalent for older versions:

db.scoreFacts.aggregate([
  { $match: {"instrumentRanges.flute.minPitch": {$gte: 0, $lte:56}} },
  { $project: {
      "_id": 1,
      "minPitch": "$instrumentRanges.flute.minPitch"
    }
  }
])

But if you can use MongoDB 4.4 or later, the first find() solution is perfect—it keeps everything within the find() method as you requested.

内容的提问来源于stack exchange，提问作者Arman