The MongoDB query language allows filtering documents based on the existence or absence of a given field with the `$exists` operator.

Is there a way, with the MongoDB syntax, and given a set `K` of allowed fields, to exclude documents that have fields not in `K` from the results, but:

* not knowing in advance which extra fields (outside `K`) can be encountered
* not using JavaScript, that is, the `$where` operator?

**Example:**

```
{
  "Some field" : "foo"
}
{
  "Some field" : "bar",
  "Some other field" : "foobar"
}

```

With the set `K = [ "Some field" ]`, only the first document is to be returned.

Note how this is not to be confused with a projection, which would return both documents but removing the extra field.