Validating a SimpleForm field within React-Admin - react-admin

Question

I'm using an old version (2.9) of the React-Admin framework and having trouble validating a SimpleForm field.
I realize upgrading to a newer version is the way to go, but that will be a while.
I'm validating a field in a SimpleForm.
My validatefunction (below) returns an integer, 0 or above.
I want the validation to pass when that value is 0, but fail when the value is > 0. Currently, the form vails validation in all cases. Instead, I want it only fail when the return value is > 0.
Is there some way to specify that using some kind of expression on the validate line in the SimpleForm below?
const MyComponent = ({ dispatch, dataProvider, ...props }) => {
return (
<React.Fragment>
<SimpleForm>
...
<ReferenceInput
source="somefield"
validate={[ required(), validatefunction ]}
>
<SelectInput optionText="name" />
</ReferenceInput>
...
</SimpleForm>
</React.Fragment>
)
}
A better approach might be for my validatefunction to return undefined when the validation passes. But for reasons outside the scope here I haven't yet gotten that working.

Answer 1

Your validateFunction should return a string if there is an error.
const validatefunction = (value) => {
const validatorValue = oldValidateFunction(value);
if (validatorValue > 0) {
return 'error this does not validate';
}
}
See https://marmelab.com/react-admin/doc/2.9/CreateEdit.html#per-input-validation-custom-function-validator for the documentation of input validators.