Querying with multiple predicates on sub-objects in a collection with MongoDB (official c# driver)

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?

I had some trouble understanding your document structure, maybe you could format it better? Also, now that I understand what you're asking for (I hope), I think you should use a different title: your question isn't about how to reach into 3rd-level objects, but rather how to apply complex predicates to a single sub-document in a collection. — Avish
– Avish, Commented Jun 27, 2011 at 9:21

Avish · Accepted Answer · 2011-06-27 09:20:12Z

3

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.

edited Jun 27, 2011 at 9:20

answered Jun 27, 2011 at 9:05

Avish

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1 Comment

Avish Over a year ago

Happy to help. I actually learned about this in the process of writing the answer, so thank you :)

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