public GroupNestedStreamable(
IStreamable <TOuterKey, TSource> source, Expression <Func <TSource, TInnerKey> > keySelector)
: base(source.Properties.GroupNested(keySelector))
{
Contract.Requires(source != null);
Contract.Requires(keySelector != null);
Source = source;
KeySelector = keySelector;
if (Source.Properties.IsColumnar && !CanGenerateColumnar())
{
properties = properties.ToRowBased();
Source = Source.ColumnToRow();
}
}