At the moment a ValidationError only returns the name of the field which doesn't validate and a "default" message not very suitable for front-end. No other info about the validation context are exposed by the error class.

My proposal is to extend ValidationError changing the behavior and content of the data attribute with a more useful payload. This payload should include the validation keyword and params (returned by Ajv validation error in default implementation).

Something like:

model/AjvValidatior.js

function parseValidationError(errors) {
  const errorHash = {};
  let index = 0;

  for (let i = 0; i < errors.length; ++i) {
    // ...
    // Here we pass a more useful payload for each key.
    // Before was: errorHash[key] = error.message;
    errorHash[key] = {
      message: error.message,
      keyword: error.keyword,
      params: error.params
    }
  }

  return new ValidationError(errorHash);
}

This is necessary to avoid nasty regex matches on front-end in order to individuate the scope of the validation error and to provide custom error messages without pains, e.g.:

MyModel
  .query()
  .patchAndFetchById(id, values)
  .catch(function(err) {
    if (err instanceof objection.ValidationError) {
     Object.keys(err.data).forEach(function(key) {
       let keyErr = err.data[key];
       if (keyErr.keyword === 'minLength')
         console.error("The field " + key + " is too short: must be at least " + keyErr.params.limit + " chars long!");
       }
     });
    }
  });

I could work on a PR for this.

For sure, it could break something so maybe a major release is required in order to include it mainstream.