MongoDB: Get just single member of array field on document - mongodb

Question

Say I have a Foo document like the following:
{
_id: 1,
bar: [{_id: 1, ...bar props}, {_id: 2, ...bar props}, {_id: 3, ...bar props)}],
... other foo props
}
How do I query the database for a single Bar, such that my result looks like:
{_id: 2, ...bar props}
Something like:
db().collection('foo').findOne({ _id: 1, {foo: _id: 2}}, {foo: 1, _id: 0})

Answer 1

Matching and projection are separate operations in MongoDB and you should also keep them separate when you are thinking (and asking) about queries.
You cannot "query for a single Bar". Queries always match documents. What you can do is find a document which contains a Bar which matches conditions, or you can find a document which contains exactly one Bar which also matches conditions, etc. In all of these cases you still get the top-level document(s) as a result.
To retrieve (only) one, several or all of the Bars in whichever documents matched your query conditions, instead of those documents, use projection (either second argument to find or $project aggregation pipeline stage).
When you are using the aggregation pipeline, you can mix $match and $project stages so that, for example, you $match to filter down documents, then $project to reduce the documents to some of their fields, then $match to further filter down the resulting documents, and so on. Still matching and projection are separate operations.