Casting an object not supported in .NET MongoDB C# Driver

Question

I am storing documents in MongoDB, represented by the following class:

public class Entity
{
    public string Id;
    public string Name;
    public List<EntityAttribute> Attributes = new List<EntityAttribute>();
}

public class EntityAttribute
{
    public string Key;
    public object Value;
    public string DataType;
}

The "Value" object on "EntityAttribute" can obviously be any type of object (string, int, float, etc), and that is the intention.

When my app attempts to perform a query using a relational operator, such as "<" or ">", it will cast the "EntityAttribute.Value" object to the appropriate type (in order to use a relational operator). The following is an example:

var results = entities.AsQueryable<Entity>()
                .Where(x => x.Type == "dogs" && 
                    x.Attributes.Any(a => a.Key == "age" && (int)a.Value > 5)).ToList();

This produces the following exception:

Unsupported where clause: ((Int32)a.Value > 5).

I am using the MongoDB CSharp Linq driver (http://docs.mongodb.org/ecosystem/tutorial/use-linq-queries-with-csharp-driver/). My assumption is that casting is not supported. Is this the case?

As a comparison, this same type of syntax (linq query) does work within RavenDB's LINQ driver, which is:

var results = session.Query<Entity>()
                        .Where(x => x.Type == "dogs")
                        .Intersect()
                        .Where(x => x.Attributes.Any(a => a.Key == "age" && (int)a.Value > 5)).ToList();

Is there any way to accomplish this, via LINQ, using the MongoDB CSharp LINQ Driver?

I can get all required queries to work using Query.EQ, Query.GT, and Query.LT, though I would rather use LINQ if possible.

Answer 1

You are correct that LINQ queries of this form (downcasting a field of type object to a more concrete class in order to do a comparison) are not currently supported by the C# driver.

I've created a JIRA ticket requesting this feature. See:

https://jira.mongodb.org/browse/CSHARP-900