Question

I’ve the following data structure:

• A
  • _id
  • B[]
    • _id
    • C[]
      • _id
      • UserId

I’m trying to run the following query:

where a.B._id == 'some-id' and a.B.C.UserId=='some-user-id'.

That means I need to find a B document that has a C document within with the relevant UserId, something like:

Query.And(Query.EQ("B._id", id), Query.EQ("B.C.UserId", userId));

This is not good, of course, as it may find B with that id and another different B that has C with that UserId. Not good.

How can I write it with the official driver?

Answer 1

If the problem is only that your two predicates on B are evaluated on different B instances (“it may find B with that ID and another different B that has C with that UserId”), then the solution is to use an operator that says “find me an item in the collection that satisfies both these predicates together”.

Seems like the $elemMatch operator does exactly that.
From the docs:

Use $elemMatch to check if an element in an array matches the specified match expression. […]
Note that a single array element must
match all the criteria specified; […]
You only need to use this when more than 1 field must be matched in the array element.

Try this:

Query.ElemMatch("B", Query.And(
  Query.EQ("_id", id),
  Query.EQ("C.UserId", userId)
));

Here’s a good explanation of $elemMatch and dot notation, which matches this scenario exactly.