Dataannotations in metadata Model

Using models created by entity framework, when I use a partial class with metadata, I cannot access the attributes added with dataannotations.
Example:

public class ClientsMetadata
{
   [NotMapped]
   public string Name { get; set; }
}
[MetadataType(typeof(ClientsMetadata))]
public partial class Clients{}

When I use:

if (property.GetCustomAttributes(typeof(NotMappedAttribute), false).Length == 0)

It does not validate the Attribute.