Show server-side validation errors after failed fo

How can I show validation messages after failed form submit? API request returns HTTP 400 'application/problem+json' response and contains violations as a list with field path.

https://tools.ietf.org/html/rfc7807#section-3

{
   "type": "https://example.net/validation-error",
   "title": "Your request parameters didn't validate.",
   "invalid-params": [ 
      {
         "name": "age",
         "reason": "must be a positive integer"
      },
      {
         "name": "color",
         "reason": "must be 'green', 'red' or 'blue'"
      }
   ]
}

I have the solution for you, I'd recommend to do it with Saga and HttpError.

First, from our dataProvider we need to throw the HttpError like this:

...
import {HttpError} from 'react-admin';
...
...

// Make the request with fetch/axios whatever you prefer and catch the error:
// message - the message that will appear in the alert notification popup
// status - the status code
// errors - the errors in key => value format, example in comment below
return fetchClient.request(config).then((response) => {
      return convertHTTPResponse(response, type, resource, params);
    }).catch(function (error) {
      throw new HttpError(error.response.data.message, error.response.status, error.response.data.errors);
    });

Then create saga like that:

import {CRUD_CREATE_FAILURE} from "react-admin";
import {stopSubmit} from 'redux-form';
import {put, takeEvery} from "redux-saga/effects";

export default function* errorSagas() {
  yield takeEvery(CRUD_CREATE_FAILURE, crudCreateFailure);
}

export function* crudCreateFailure(action) {
  var json = action.payload;
  // json structure looks like this:
  // {
  //     username: "This username is already taken",
  //     age: "Your age must be above 18 years old"
  // }
  yield put(stopSubmit('record-form', json));
}

Please make sure the errors (json) is in the format like in the example above!

Then insert the saga in the componenet:

import errorSagas from './sagas/errorSagas';
...
...

<Admin
        customSagas={[ errorSagas ]}
        loginPage={LoginPage}
        authProvider={authProvider}
        dataProvider={dataProvider}
      >

Boom! it works