Custom error message for loopback 4 model schema validation

Question

In my model, I have defined a model schema. One of the property is the name which is a required field and minimum character should be of length 5.

  @property({
    type: 'string',
    required: true,
    jsonSchema: {
      title: 'name',
      minLength: 5,
      maxLength: 64,
    },
  })
  name: string;

But the issue is the error message I get if validation for name fails is (i.e. I pass less than 5 characters),

{
    "error": {
        "statusCode": 422,
        "name": "UnprocessableEntityError",
        "message": "should NOT be shorter than 5 characters"
    }
}

How can I define a custom error message?

score 4 · Answer 1 · answered May 15 '20 at 11:05

I found it in loopback4 documentation that you can define customer error message in jsonSchema object itself. Please find below code,

@property({
   name: 'name',
   description: "The product's common name.",
   type: 'string',
   // Specify the JSON validation rules here
   jsonSchema: {
      maxLength: 30,
      minLength: 10,
      errorMessage: 'name must be at least 10 characters and maximum 30 characters',
   },
})

Goto https://loopback.io/doc/en/lb4/Model.html#id-properties and search for section,

Validation Rules

You will get this property to set custom error message. I checked and its working fine!

Thanks!!

Thanks For the comment. Did not really work for me but I will try updating my @loopback/rest module and try again. — prisoner_of_azkaban, May 15 '20 at 14:09

Miroslav Bajtoš · Answer 2 · 2020-05-15T14:20:40.757

Hello from the LoopBack team

I am able to reproduce your scenario using LoopBack's Todo example app. Here is the full error response I am seeing:

{
  "error": {
    "statusCode": 422,
    "name": "UnprocessableEntityError",
    "message": "The request body is invalid. See error object `details` property for more info.",
    "code": "VALIDATION_FAILED",
    "details": [
      {
        "path": "/name",
        "code": "minLength",
        "message": "should NOT be shorter than 5 characters",
        "info": {
          "limit": 5
        }
      }
    ]
  }
}

As you can see, the details field contains machine-readable information that can be used by clients to render a human-friendly error message:

The path field is pointing to the (deep) property that did not pass validation (name).
The code field contains the validation rule that was violated (minLength).
info.limit provides additional information about the validation rule.

The error message you posted does not look familiar to me. Did you configure a custom error handler in your application (see Handling errors in Sequence)? Are you perhaps using an older version of @loopback/rest? Can you please update your LoopBack dependencies to their latest versions and see if you get more helpful error messages?

Yes, I have indeed configured a custom error handler. That's why there are no details in my response. — prisoner_of_azkaban, May 15 '20 at 14:02

score 0 · Answer 3 · answered May 15 '20 at 15:17

To customize the error message, there's a section in https://loopback.io/doc/en/lb4/Validation-REST-layer.html#customize-validation-errors related to this.

You can also see a running app in: https://github.com/strongloop/loopback-next/blob/master/examples/validation-app/src/sequence.ts.

Custom error message for loopback 4 model schema validation

3 Answers3