Prevent New Child Record without triggers

Question

I am trying to find the best method, that will work both in classic and lightning, to prevent a user from creating a more than 1 child record from a parent object; e.g pressing 'New' on a related list.

1. Validation Rule / Triggers : These occur after the user presses 'Save'.. so I dont like this because it would mean the user has already spent time filling out details for the child record.
2. Javascript button: Detail page button to redirect to new child record only if a rollup summary field returns 0 ... but javascript buttons are not supported in lightning.

I think the best solution would seem to be a detail page button to a custom visualforce page with custom controller to handle redirect to the existing child record or show a page with an error message and ask wher they'd like to be redirected.

Answer 1

If you are creating child record from parent record's detail page then create a recordtype on the parent object and create a separate page layout where you can remove the New button of Child object's related list. Map this recordtype with the page layout.

When you create a child record then update the recordtype of Parent record. This way, user will not see the New button in the Parent record's related list.

Also, put a validation rule, which is based on Child count which you have mentioned, just in case, if child record is created from API or data load.

This is most effective OOTB approach.