MongoDB投影指定嵌套字段失败，求解决思路

Let's break down what's going wrong and how to fix it:

1. The Array Index Mistake #

Your biggest issue is the invalid path for the stringValue field. In your sample document, RULES is an array with one element (array indices start at 0, not 1). Using _id.RULES.1.0._Rules.stringValue targets a non-existent element, which throws off the entire projection.

2. Corrected find() Query #

Use the correct array index (0) and remove the duplicate CURRENTSTATE entry in your projection:

db.results.find({}, {
  "_id.RULES.0._Rules.stringValue": 1,
  "_id.MYOPERATION": 1,
  "_id.CURRENTSTATE": 1
})

This will return only the desired subfields nested under the _id object:

{
  "_id": {
    "RULES": [{"_Rules": {"stringValue": "Y_RULES"}}],
    "MYOPERATION": 1,
    "CURRENTSTATE": "PROPOSED"
  }
}

3. Flatten Fields to Top-Level (Optional) #

If you prefer to extract these fields as top-level properties instead of keeping them nested under _id, use the aggregation framework's $project stage:

db.results.aggregate([
  {
    $project: {
      ruleStringValue: "$_id.RULES.0._Rules.stringValue",
      myOperation: "$_id.MYOPERATION",
      currentState: "$_id.CURRENTSTATE",
      _id: 0 // Exclude the original _id field if you don't need it
    }
  }
])

Result:

{
  "ruleStringValue": "Y_RULES",
  "myOperation": 1,
  "currentState": "PROPOSED"
}

Why Your Original Query Failed #

The invalid array index (1.0) meant MongoDB couldn't resolve that field, but since you were targeting subfields of _id (which is included in results by default), it ended up falling back to returning the entire _id object (and any other top-level fields present). Fixing the index tells MongoDB exactly which fields to include in the projection.

内容的提问来源于stack exchange，提问作者HVA