ArgumentNullException with ElemMatch

Question

The following Entity is given:

public class MyClass
{
    public MyClass()
    {
        Aliases= new List<string>();
    }

    public Guid Id { get; set; }

    public string Name { get; set; }
    public List<string> Aliases{ get; set; }
}

I want to search by name or by any matching alias using the following query:

Query.Or(
    Query<MyClass>.Matches(a => a.Name, request.Name),
    Query<MyClass>.ElemMatch(a => a.Aliases, query => query.Matches(alias => alias, request.Name))
)

This query works well as long as myClass.Aliases.Count > 0! But as soon as its empty i get the following exception:

System.ArgumentNullException was unhandled by user code
  HResult=-2147467261
  Message=Value cannot be null.
Parameter name: name
  Source=MongoDB.Driver
  ParamName=name
  StackTrace:
       at MongoDB.Driver.Builders.Query.Matches(String name, BsonRegularExpression regex)
       at MongoDB.Driver.Builders.QueryBuilder`1.Matches(Expression`1 memberExpression, BsonRegularExpression regex)
       at My.Namespace.MyService.<>c__DisplayClassf.<Get>b__d(QueryBuilder`1 builder) in c:\Dev\....cs:line 148
       at MongoDB.Driver.Builders.QueryBuilder`1.ElemMatch[TValue](Expression`1 memberExpression, Func`2 elementQueryBuilderFunction)
       at MongoDB.Driver.Builders.Query`1.ElemMatch[TValue](Expression`1 memberExpression, Func`2 elementQueryBuilderFunction)

How can i workaround this limitation/bug?

Answer 1

ElemMatch doesn't filter the results like a where clause, it only limits the array returned to one matching item, or none. You need to first filter the documents, and then limit the array inside it.

You should use Eq like this:

Query.Or(
    Query<MyClass>.EQ(a => a.Name, request.Name),
    Query<MyClass>.EQ(a => a.Aliases, request.Name));

Lets assume that request.Name = "bar" and the query should look like this:

{
    "$or" : [
    {
        "Name" : "bar"
    },
    {
        "Aliases" : "bar"
    }]
}

MongoDB knows that Aliases is an array and it will "search" for the name and not "just compare"