Doctrine ORM: Updating Associations of an Entity - Fetch all Entities from DB?

Question

Im new to Doctrine, and using ORM's.

I want to update the join table of an entities association in Doctrine ORM.

I have 2 Entities with a join table for a Many-toMany association

Zoos Table

id	zoo_name
1	London Zoo
2	New York Zoo

Animals Table

id	animal_name
1	Lion
2	Bear
3	Donkey
4	Otter

Zoos_Animals Join Table

zoo_id	animal_id
1	1
1	3
2	1
2	2
2	3
2	4

Historically, when not using an ORM I would post the ID's of all the animals a zoo has, then save in the following way:

1. DELETE FROM zoo_animals WHERE zoo_id = 1
2. FOREACH $_POST['updated_animals'] as $animal_id {
     INSERT INTO zoo_animals ($_POST['zoo_id'], $animal_id
   }

I've simplified this to get my point across, (I would sanitize data etc..)

But when using Doctrine, it seems that even though I have all the information I need in $_POST (an array of animal_id's), in order to update zoo_animals for zoo 1, I need to:

1. On save run a DQL query which gets all zoo info out along with it's zoo_animals
2. Build a new ArrayCollection for the Zoo's $animals property by fetching all Animals posted in $_POST['updated_animals']
3. $zoo->setAnimals($fetchedAnimals) to the ArrayCollection we just fetched
4. Flush

All I need in my join table is the animal ID's, so it seems really in-efficient that I have to fetch the entire Animal entity for each zoo_animal from the Animals table in order to update the association.

Am I missing something - is there another way to update the join table? - maybe I shouldn't bother recreating the Zoo entity and instead should just deal directly with the join table somehow?

I did also try not fetching the Animals from DB for each $_POST['updated_animals'] and instead just create the Animal with the bare minimum data needed (which I have)

$updatedAnimals = new ArrayCollection();
foreach ($_POST['updated_animals'] as $id){
   $animal = (new Animal())->setId(id);
   $updatedAnimals->add($animal);
}

But when I Flush() I get the following error

A new entity was found through the relationship 'Zoo#animals' that was not configured to cascade persist operations for entity

..basically doctrine doesn't recognise the Animals I created from posted animal ID's, even though they are in the animals table.

As I say, I'm new to using ORM's but all this re-fetching from the DB in order to update seems very inefficient as so I suspect Im doing something wrong.