Query with filter builder on nested array using MongoDB C# driver

Question

Consider the following object structure stored as documents:

public class Foo
{
    public string Id { get; set; }
    public ICollection<FooBar> Bars { get; set; }

    // ...
}

public class FooBar
{
    public string BarId { get; set; }

    // ...
}

Using a LINQ-style query with the driver I can Find all Foo that contain a FooBar BarId like this:

var foos = await m_fooCollection.Find( f => f.Bars.Any( fb => fb.BarId == "123") ).ToListAsync();

How can I achieve this same query using the FilterDefinitionBuilder instead of the in-line LINQ on Find?

`.Any` on a `ICollection` is part of the `Enumerable` class in the `System.Linq` namespace, right? — kspearrin, Jun 22 '15 at 17:35
Indeed, but that method will never be called. The driver translates the expression to a mongo query. — i3arnon, Jun 22 '15 at 17:39
Ok, makes sense. I guess I was more trying to express the fact that I am using LINQ style syntax rather than actual LINQ. — kspearrin, Jun 22 '15 at 17:42
Yes, it doesn't really matter. Just a note to future googlers. — i3arnon, Jun 22 '15 at 17:44

score 34 · Accepted Answer · answered Jun 22 '15 at 17:25

34

The query you need to perform uses the $elemMatch query operator.

So, this query using a lambda expression

var findFluent = collection.Find(f => f.Bars.Any(fb => fb.BarId == "123"));

Is equivalent to this query using the FilterDefinitionBuilder:

var findFluent = collection.Find(Builders<Foo>.Filter.ElemMatch(
    foo => foo.Bars, 
    foobar => foobar.BarId == "123"));

answered Jun 22 '15 at 17:25

i3arnon

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What if I have BarNumber property instead of BarId and there are 2 BarNumber with same value. How can I get both of them? – utaco Jan 26 '17 at 22:57
@i3arnon, this returns all the `Foo` objects that matches the criteria with each object having ALL its `FooBar` children. What if I need to filter those children objects too (with `BarId = "123"`) so not all FooBar will be present in result? – benjamin54 Aug 02 '17 at 11:14
@benjamin54 I don't think you can do that with a "simple" query. I think you need to use the aggregation framework, but I'm not sure. Maybe you'd be better posing another question... – i3arnon Aug 03 '17 at 08:49
Hey, if `Foo` object has a dateTime property, which I want to find by date greaterorequal param, how would you achieve that? Doing like so doesnt work -> var findFluent = collection.Find(Builders.Filter.ElemMatch( foo => foo.Bars, foobar => foobar.TimeCreate >= timeParam)); – Ben Mar 01 '20 at 14:17
what if I need to compare any entire child object like this? Like imagine changing the last part of your query to ... foobar => foobar.child == someobject)) Is this possible? – Urasquirrel Apr 14 '21 at 00:20
1

@benjamin54 i am looking for the same answer that you asked. I am late here but looking forward to hear form you! Please let me know if we can do that using FilterDefinitionBuilder ? otherwise projection is the option to do it. – Rizwan Shakir Oct 19 '21 at 17:00

Query with filter builder on nested array using MongoDB C# driver

1 Answers1