whereHas<TChild> method
Keeps parents that have at least one matching related row.
Correlated EXISTS, not a join: a customer with ten orders is still
one customer. The handle's where/withTrashed/then filters are the
same snapshot include would load.
Implementation
S whereHas<TChild>(
MssqlRelation<TRow, TChild> Function(TFields fields) relation,
) {
final rel = relation(fields);
return where((_) => MssqlExistsSubquery(_existsQuery(rel), negated: false));
}