MongoDB projection does not work for nested properties when the property name has a digit - Stack Overflow

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I have weather documents like this:

{
 "_id": "some_id",
 "temp_f": 65.8,
 "current_observation": {
   "rain_day_in": 0,
   "soil_moisture_1": 0
   "soil_moisture_1_day_high": 0
 }
 "date": "12/23/2021",
 "date_and_time": "2021-12-23T20:34:21",
 ... many more properties here
}

Running the following find command (find the most recent document and flatten the properties):

db.getCollection('weather').find({}, {
    "temp_f": 1,
    "rain_day_in": "$current_observation.rain_day_in",
    "soil_moisture_1": "$current_observation.soil_moisture_1",
    "soil_moisture_1_day_high": "$current_observation.soil_moisture_1_day_high",
    "date": 1,
    "_id": 0
}).sort({
    date_and_time: -1
}).limit(1);

I get the following result:

[
  {
    "temp_f": 66.6,
    "date": "03/12/2025",
    "rain_day_in": 0
  }
]

It returns all included properties except soil_moisture_1 and soil_moisture_1_day_high. Both have a digit in the name. Several other properties have a digit as part of the property name, and none of them are returned in the result if I include them. Is that digit causing the issue? How can I fix the command to get those properties in the result?

Using MongoBD v7.0.15

I have weather documents like this:

{
 "_id": "some_id",
 "temp_f": 65.8,
 "current_observation": {
   "rain_day_in": 0,
   "soil_moisture_1": 0
   "soil_moisture_1_day_high": 0
 }
 "date": "12/23/2021",
 "date_and_time": "2021-12-23T20:34:21",
 ... many more properties here
}

Running the following find command (find the most recent document and flatten the properties):

db.getCollection('weather').find({}, {
    "temp_f": 1,
    "rain_day_in": "$current_observation.rain_day_in",
    "soil_moisture_1": "$current_observation.soil_moisture_1",
    "soil_moisture_1_day_high": "$current_observation.soil_moisture_1_day_high",
    "date": 1,
    "_id": 0
}).sort({
    date_and_time: -1
}).limit(1);

I get the following result:

[
  {
    "temp_f": 66.6,
    "date": "03/12/2025",
    "rain_day_in": 0
  }
]

It returns all included properties except soil_moisture_1 and soil_moisture_1_day_high. Both have a digit in the name. Several other properties have a digit as part of the property name, and none of them are returned in the result if I include them. Is that digit causing the issue? How can I fix the command to get those properties in the result?

Using MongoBD v7.0.15

• mongodb

Share Improve this question Follow asked Mar 12 at 15:15 MiguelMiguel 1,73411 gold badge2929 silver badges3232 bronze badges 2

• 2 Cannot reproduce. Projection works as expected, here: mongoplayground/p/JNhbhhKFvAP. It's possible that the first document (or some documents) don't have that field, so they are being skipped in the output. With sort and limit 1, you only see the first one. Add a filter for { "_id": "some_id" } and see if documents which have those fields have the correct output. – aneroid Commented Mar 12 at 15:29
• 1 Thanks, @aneroid. It seems like we have a faulty sensor. Some documents have the properties, and some others don't. Nothing wrong with MongoDB. – Miguel Commented Mar 12 at 16:30

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1 Answer 1

Sorted by: Reset to default 1

Cannot reproduce the issue you're facing with your example data. Projection works as expected, here: https://mongoplayground/p/JNhbhhKFvAP.

It's possible that the first document (or some documents) don't have those fields, so they are being skipped in the output. With sort and limit 1, you only see the first document. Add a filter for { "_id": "some_id" } and see if documents which have those fields have the correct output.

If you always want those fields in the output, then use $ifNull in the projection and provide a good default, like 0 or null or -1000:

db.getCollection('weather').find({},
{
  "temp_f": 1,
  "rain_day_in": {
    $ifNull: ["$current_observation.rain_day_in", -1000]
  },
  "soil_moisture_1": {
    $ifNull: ["$current_observation.soil_moisture_1", -1000]
  },
  "soil_moisture_1_day_high": {
    $ifNull: ["$current_observation.soil_moisture_1_day_high", -1000]
  },
  "date": 1,
  "_id": 0
}).sort({
  date_and_time: -1
}).limit(1);

Mongo Playground with more documents

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