@Override
    public FieldPredicate<ENTITY> equal(String value) {
        requireNonNull(value);
        return new ReferenceEqualPredicate<>(this, value);
    }
@param <ENTITY>     the entity type
@param <D>          the database type
@param <FK_ENTITY>  the foreign entity type

@author  Per Minborg
@author  Emil Forslund
@since   2.2.0
runtime-parent/runtime-field/src/main/java/com/speedment/runtime/field/internal/StringForeignKeyFieldImpl.java