Mongodb C# driver return only matching sub documents in array

Question

I have a document in this format:

{
    _id: ...,
    myArray: [{other: stuff}, {other: stuff}, ...],
    ...
}

I want to find elements that match certain things, like the _id or fields value from the sub-documents in myArray.

I want to return the documents, but with a filtered MyArray where only the matching sub-documents are present.

I tried to do a projection and include the matched elements like this:

_mongoContext.myDocument
    .Find(x => x.id == id & x.myArray.Any(y => myList.Contains(t.other)))
    .Project<myModel>(Builders<myModel>.Projection.Include("myArray.$"))

This, I think, should only return the first element that matched in myArray instead of all documents, which is not what I want (I want all sub-documents that match the query to be present in the returned document).

And anyway it did not even work, I'm getting a positional projection does not match the query document error. Maybe it's because I'm not using FindOne?

In any case, how can I achieve what I'm looking for? (See question in bold)

Answer 1

Typically you need to use $filter in Aggregation Framework to filter nested array. However there's an easier way to achieve that using MongoDB .NET Driver and IQueryable interface.

Considering simplest model:

public class MyModel
{
    public string _id { get; set; }
    public IEnumerable<MyNestedModel> myArray { get; set; }
}

public class MyNestedModel
{
    public string other { get; set; }
}

and following data:

var m = new MyModel()
{
    _id = "1",
    myArray = new List<MyNestedModel>() {
        new MyNestedModel() {  other = "stuff" },
        new MyNestedModel() { other = "stuff" },
        new MyNestedModel() { other = "stuff2" } }
};

Col.InsertOne(m);

you can simply call .AsQueryable() on your collection and then you can write LINQ query which will be translated by MongoDB driver to $filter, try:

var query = from doc in Col.AsQueryable()
            where doc._id == "1"
            select new MyModel()
            {
                _id = doc._id,
                myArray = doc.myArray.Where(x => x.other == "stuff")
            };

var result = query.ToList();

EDIT:

Alternatively you can write $filter part as a raw string and then use .Aggregate() method. Using this approach you don't have to "map" all properties however the drawback is that you're losing type safety since this is just a string, try:

var addFields = BsonDocument.Parse("{ \"$addFields\": { myArray: { $filter: { input: \"$myArray\", as: \"m\", cond: { $eq: [ \"$$m.other\", \"stuff\" ] } }  } } }");

var query = Col.Aggregate()
               .Match(x => x._id == "1")
               .AppendStage<MyModel>(addFields);

$addFields is used here to overwrite existing field.