Reverse side association mapping can not lazy loaded

要特别注意一对一，一对多，等映射关系的 reverse side。因为 reverse side 没有外键，doctrine 无法创建代理对象。

Arbitrary Join

语法和sql类似，把 join - on 改成 join with

Arbitrary Join

Remove OneToMany Collections

If doctrine collection object is owning side, then call collection->clear() method will also remove the items from
database.

But OneToMany association is always the reversed side, so It not work.

Set orphanRemoval flag on OneToMany attribute can fix this, but I don't like it. Because it is implicit
and hard to test. orphanRemoval delete items one by one:

        if ($this->association['type'] & ClassMetadata::TO_MANY &&
            $this->association['orphanRemoval'] &&
            $this->owner) {
            // we need to initialize here, as orphan removal acts like implicit cascadeRemove,
            // hence for event listeners we need the objects in memory.
            $this->initialize();

            foreach ($this->collection as $element) {
                $uow->scheduleOrphanRemoval($element);
            }
        }

Note the flag ClassMetadata::TO_MANY, it is 12, combine bits 8 and 4, dump it at runtime, it actually maybe 4, so set orphanRemoval maybe not work.

The same performance as delete one by one in a loop, so here is my way: explicit delete each items using EnittyManager::remove() method, easier to unit test, stable, and the same performance.