Query a <map> collection in nhibernate

Question

Given a class...

public class Message
{
    /// key: User, value: folder name
    private readonly IDictionary<User, string> _folders = new Dictionary<User, string>();
    public virtual IDictionary<User, string> Folders { get { return _folders; } }
}

<map name="Folders" access="field.camelcase-underscore">
    <key column="Message_id" />
    <index-many-to-many class="User" column="User_id" />
    <element column="FolderName" type="System.String" />
</map>

Is it possible to

1. Get all messages for a user
2. Get all messages for a user with a specific folder name

Thanks