default Optional<? extends ForeignKey> findForeignKey(String id) {
        return foreignKeys().filter(child -> child.getId().equals(id)).findAny();
    }
Locate the {@link ForeignKey} child with the specified id if it exists,
else return an empty {@code Optional}.

@param id  the {@link HasId#getId()} of the foreign key
@return    the child found or an empty {@code Optional}
runtime-parent/runtime-config/src/main/java/com/speedment/runtime/config/Table.java