Mongodb nested query doesn't give result when operators are involved

Question

Rrunning the following snippet I would expect all of the finds to return the same result, however the middle on does not find any elements. Why doesn't the second case find anything and is there any way to use the nested element notation instead of dot notation with the mongodb operators?

from pymongo import MongoClient
client = MongoClient("mongodb://localhost:27017")
db = client['db']
collection = db["test_collection"]
collection.insert_one({"data": {"foo": "bar"}})

# {'_id': ObjectId('633aad8b587bc8057e240c96'), 'data': {'foo': 'bar'}}
print(collection.find_one({"data": {"foo": "bar"}}))

# None
print(collection.find_one({"data": {"foo": {"$in": ["bar"]}}}))

# {'_id': ObjectId('633aad8b587bc8057e240c96'), 'data': {'foo': 'bar'}}
print(collection.find_one({"data.foo": {"$in": ["bar"]}}))

user20042973 · Accepted Answer · 2022-10-03 13:03:04Z

1

Why doesn't the second case find anything?

The second query is using embedded document syntax to search for documents. As you mentioned, dot notation is the alternative syntax. Both are documented here.

Each approach has different semantics. The second query that you are asking about is searching for a document that has exactly this object for its data field:

{
  foo: {
    $in: [
      "bar"
    ]
  }
}

Check out this playground demonstration which shows this behavior with more example documents.

Of course that's not what you want which is one of the reasons the alternative dot notation syntax is available.

is there any way to use the nested element notation instead of dot notation with the mongodb operators?

As written, the answer to that question is: no. As we explored above, querying using embedded syntax results in the following semantics (taken from the documentation):

Equality matches on the whole embedded document require an exact match of the specified <value> document, including the field order.

Usage of query operators, which aren't intended for this syntax, is going to result in them getting interpreted as literal field names when searching the collection. But even your first example query probably isn't generally useful as the document embedded in data could not have any other fields as demonstrated in this playground

answered Oct 3, 2022 at 13:03

user20042973

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4 Comments

user20042973 Over a year ago

I would separately be curious about the motivation for the question, especially the latter one. Was this just curiosity that came from the discovery of the different syntax options?

Jovasa Over a year ago

I originally tried to use the nested query because it seemed intuitive to me, but since it wasn't working I read this mongodb.com/docs/manual/reference/method/db.collection.find/… and nothing on that makes me think that the two different notations would behave differently, which lead to my confusion and this question.

user20042973 Over a year ago

Oh wow that is confusing/misleading!

user20042973 Over a year ago

Ah, rereading that section is specifically about projection syntax. What this question is concerned with is semantics for querying syntax, which is what the link in this answer points toward. Still a bit nuanced, but ultimately I think everything here is 'technically correct'

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