toLookup_<TKey> method
: groups the source by keySelector.
The result is iterable: iterate to get
IGrouping<TKey, T> entries (one per key).
For random access by key (.containsKey, ``),
use Queryable.toLookup_ on a SQL-backed
queryable.
Implementation
ILookup<TKey, T> toLookup_<TKey>({required Expr keySelector}) {
final LambdaExpr lambda = _requireLambda('toLookup_', keySelector);
return buildLookup<T, TKey>(this, lambda);
}