/// <summary>
/// Subscribe to a specific set of columns/fields for a given row key
/// </summary>
/// <param name="rowKey">Row key to index on</param>
/// <param name="columns">Set of columns to subscribe to</param>
/// <returns>Observable value representing the subscription</returns>
public IObservable<IConstituentRow> Subscribe(object rowKey, IEnumerable<IColumn> columns)
{
var compositeSubscription = new CompositeSubscription(columns);
foreach (var table in columns.Select(c => c.Table).Distinct())
compositeSubscription.AddTable(table, Subscribe(rowKey));
return compositeSubscription;
}