MongoDB C# driver - how to query a property on an array of subdocuments

Question

Let's suppose I have the following document structure in MongoDB.

{
    _id: ####,
    Ancestors: [
        { _id: 1, Name: "asdf" },
        { _id: 2, Name: "jkl;" },
        ...
    ]
}

I want to find every document that contains an Ancestor where _id of the Ancestor is 2.

I can run this query in the mongo shell using this: db.projects.find({"Ancestors._id": 2})

I can also run this query using the official C# driver using this: Query.EQ("Ancestors._id", new BsonInt32(rootProjectId)).

Here are my POCOs; the actual classes I'm using have more properties than this, but I didn't want to clutter up the question with unnecessary details:

public class Project
{
    public int Id { get; set; }
    public List<ProjectRef> Ancestors { get; set; }
}

public class ProjectRef
{
    public int Id { get; set; }
    public string Name { get; set; }
}

My question is: How can I write a strongly-typed query using the C# driver so that I don't have to pass in "Ancestors._id" as a string? I want to be able to do something like Query<Project>.EQ(p => p.Id, rootProjectId) so that I can instead use a member expression and let the class mappings tell the driver that it should use "Ancestors._id".

Answer 1

ElemMatch is your friend in this case.

try the following:

var ancestorsQuery = Query<ProjectRef>.EQ(pr => pr.Id, rootProjectId);
var finalQuery = Query<Project>.ElemMatch(p => p.Ancestors, builder => ancestorsQuery));

use finalQuery within a Find command on the Projects collection.