Need Help Understanding Doctrine 2's ArrayCollection::clear()

I just don开发者_如何学运维't really get this part

Say you clear a collection of tags by calling $post->getTags()->clear(); and then call $post->getTags()->add($tag). This will not recognize tag being already added before and issue two database calls.

What 2 database calls will be issued? Delete all tags of the post then add one? Thats what I'd expect? Or will it be something else?

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Did you try to call after the clear?

$entityManager->flush();  

If this don't help try to remove one by one in foreach.