When used in the $match stage, these two aggregation stages return different documents:

  $expr: {
    $eq: ["$cheese", undefined]
  }

{
  cheese: {$eq: undefined}
}

The field cheese is not a property of any document in the collection.

In the second query, it returns all documents, which is expected behavior.

In the first query, it returns no documents.

What explains the difference?

CodePudding user response：

Using $expr MongoDB will return always false when compare with undefined as is not a valid value to compare with.

But without $expr mongo uses MongoDB query language (like a find query) which will compare if the field exists.

Is that the reason because using the first query you do not receive anything (the comparsion is always false) and the second query return all documents (the field does not exist so the comparsion is always true).