/// <summary>
/// Add a model object's ID as a first ID, mapped to a specified
/// second. The category to store it under will be automatically determined
/// by the ModelObject's type.
/// </summary>
/// <param name="mObject"></param>
/// <param name="secondID"></param>
public void Add(Unique mObject, TSecondID secondID)
{
string category = GetCategoryForType(mObject.GetType());
TFirstID firstID = ExtractFirstID(mObject);
Add(category, firstID, secondID);
}