How to match a single property value in MongoDB

September 13, 2022

Suppose a collection contains the following 3 documents:

[
  { "_id": 1, "prop": 1 },
  { "_id": 2, "prop": 4 },
  { "_id": 3, "prop": [1, 2, 3] }
]

The query { $match: { prop: 1 } } returns 2 documents, namely 1 and 3! I would have expected it to only return 1.

Is this behaviour documented somewhere or is it a bug?
How could one formulate the query to mean strict equality (as opposed to equality or array-contains)?

>Solution :

I think that MongoDB will always try to match against both scalars and arrays, unless you explicitly rule out the latter:

{ $match : { prop : { $eq : 1, $not: { $type : 'array' } } } }

It doesn’t seem to be explicitly documented, but it’s implied in the documentation because the syntax for querying scalars for a particular value is the same as the syntax for querying arrays.