The Problem Recently, while working on a project using React-Admin, I encountered a frustrating issue. I had an Edit component with a TabbedForm set to pessimistic mode, and a custom data provider based on React-Admin’s simple REST provider. Everything worked fine until the server-side validation kicked in.
The validation rule was straightforward: it ensured that no two entities shared the same name. If the rule was violated, the server would correctly return a 400 error with a message, which I displayed in a toast notification.
