How to save a recordset without overwritting existing records with Cakephp 3 - database

Question

Is there an easy way to save a recordset, i mean multiple records, but only the "new ones"?
I have a table users and a users form in the view. First field you must enter is passport number, so if user already exists in database the rest of the form will be auto completed and disabled to prevent changes but if passport dont exist then you have to enter all data. As anyone can change those existing users data controls from the browser even if they are disabled, i want to make sure only new records are saved in database. First of all i thought i could find again in database and delete existing users from recordset before save, but i wonder if there is a more elegant approach.
Ty in advance.
I write this here, cause comments are too short:
Thank you for your answer, André. I'm sorry if i didnt explain perfectly, but the form is done by disabling all controls except passport and if passport dont exist (i check it on passport focusout) then the rest of controls are set to enabled. I mean, that is already done. The question was only about the saving.
The validation method you talk about, well i'm actually validating all the controls in the form and i must disable the 'unique' rule so a user can link an existing user to the current bill, otherwise it will fail validation on submit and it wont allow the user to proceed (i did this because it happened to me when testing). The actual setting is much more complicated: the form belongs to a model (bills) which is associated with 4 other models and 2 of those relationships are many to many, bills_users and bills_clients, where users are the persons who do the job and clients pay for it, but I was trying to make the question easier. Anyway, what I am looking for is, in fact, some kind of saving setting which I can't find. In documentation I found "When converting belongsToMany data, you can disable the new entity creation, by using the onlyIds option. When enabled, this option restricts belongsToMany marshalling to only use the _ids key and ignore all other data." The first half of the sentence was promising, but the explanation says different, and I actually tried it without success.

Answer 1

First:
Is there an easy way to save a recordset, i mean multiple records, but only the "new ones"?
Yes there is you can validate it in model, something like this:
public function buildRules(RulesChecker $rules)
{
$rules->add($rules->isUnique('passportNumber'));
return $rules;
}
This will prevent to save a duplicate passport number register, but you can different.
I have a table users and a users form in the view. First field you must enter is passport number, so if user already exists in database the rest of the form will be auto completed and disabled to prevent changes but if passport dont exist then you have to enter all data.
There is two different ways to do this:
First you have your form, you develop an ajax function when you fill the first field (passport number) this ajax function do a request to your controller to search for a passport with that number if find something get data and fill others fields and turn them just readable, else just nothing and let user fill the fields by himself
second add a step-by-step where you first do a form to try find by pass number, user fill this only field with a number then submit, on submit this search for a record, if find fill the entire next step fields, else the next step will be the rest of form with the fields to be filled.
This may help you too: https://book.cakephp.org/3.0/en/orm/validation.html
Tell me how you decided to do :)