Update custom_constraint.rst

[DonCallisto]

Q	A
Doc fix?	yes
New docs?	no
Applies to	>=2.3
Fixed tickets

You MUST specify an alias because

//Symfony\Bundle\FrameworkBundle\Validator\ConstraintValidatorFactory.php
[...]
// $name will be a FQCN and will not be present in $this->validators array
$name = $constraint->validatedBy();

if (!isset($this->validators[$name])) {
    $this->validators[$name] = new $name(); //LOOK HERE; IF I DON'T SPECIFY THIS ALIAS, I WILL END HERE: NO SERVICE!
} elseif (is_string($this->validators[$name])) {
    $this->validators[$name] = $this->container->get($this->validators[$name]);
}

[...]

So if custom validator is a service that needs to be taken from Container, there is no valid alternative to alias: it should be mandatory (and should be reported onto documentation) as you can see from this factory.

[marcoalbarelli]

+1
Also this is affecting also past versions of Symfony, not just the 3.x ones

[DonCallisto]

@marcoalbarelli yes, is true: I forgot to report this.

[marcoalbarelli]

I'd add that alias must match what is returned by the validateBy method and that it must not necessarily match the service id

[marcoalbarelli]

Just to be on the nitpicking side of things: it should be noted that if the alias value matches the FQCN of the *Validator class the validatedBy override can be omitted

<service id="somebundle.file.validator.base_validator" class="SomeBundle\Validator\Constraints\BaseValidator">
      <tag name="validator.constraint_validator" alias="SomeBundle\Validator\Constraints\BaseValidator"/>
      <!-- rest of the service definition -->
</service>

[DonCallisto]

@marcoalbarelli yes but I suppose that's not a Symfony standard anymore that way :)

[marcoalbarelli]

I didn't find any explicit constraint or standard on what should be used as an alias in "normal" services definitions, but since the alias should match a service id, which in turn has to be a valid yaml key, and since a FQCN is not a valid yaml key, I think services ids are de-facto standardized to what can be used as a valid yml key.

In this context though the term alias denotates a different thing. It doesn't match a yml key, but the output of a function.
I agree that this is a grey area and some clearing up should be performed.

Just for reference: the xsd for the service tag only says that the id property must be a string.
More strict checking is surely possible in xsd's (check the boolean type on that file for instance) but none is configured

[marcoalbarelli]

I can confirm that there is no standard in service id naming and since 3.1 any utf8 string will be considered as valid.
I just got a PR rejected for this reason

[javiereguiluz]

The current docs state that alias is optional. We need a definitive decision about this (it's required/optional/convention, it's for 2.x/3.x, etc.):

[javiereguiluz]

After checking the source code of the validator component, I'm closing this in favor of #7489. But I'm not 100% sure, so I've opened symfony/symfony#21610 too. Thanks!