CakePHP: how to delete haveMany associations with saveAll

I have a model model1 which hasMany model2. How can I remove all the model2s from a specific model1, e.g. the one with ID 1234? This means, in SQL,

UPDATE
    model2
SET
    model1_id=NULL
WHERE
    model1_id=1234;

But how to formulate this the CakePHP wa开发者_运维知识库y? I tried model1->saveAll with the following array as argument:

Array
(
[Model1] => Array
    (
        [id] => 1234
    )

[Model2] => Array
    (
    )
)

Which means, Model2 just set to a empty array. But this doesn't work. Now, how can I "un-associate" objects2 which belong to another specific object1 in a hasMany (seen from object1 side) relationship?

Try using updateAll($fields, $conditions)

$this->Model2->updateAll(array('model1_id' => null), array('model1_id' => 1234));

Here's the CakePHP Docs about this.