[Form][Validator] Add new unique entity validation on form type

[94noni]

Hello,

Where we have entity without validation on it, and want validation on the form type, this can be valuable I think

NoteI did not found such info on the doc so after slack discuss, I got the answer and thus proposed this doc

[javiereguiluz]

Thanks for improving this code example. We merged it in 5.4 and up.

[94noni]

@javiereguiluz thx! but watch out that I use a php8 feature

[xabbuh]

I am not completely convinced of this change. Without any comment this may now look like you will have to add the UniqueEntity constraint to the entity and the form.

[94noni]

@xabbuh thx for commenting, I would be glad to PR adding a comment but I am unsure of what kind of comment you have in mind, may I ask you more details?

like, the one in the PR desc for example?

[xabbuh]

@94noni Sorry for being late with my response, but can you explain why it was necessary to add the constraint option to the form at all in your case and why the example as it was before did not work?