custom-validator | Gin Gonic Binding validation error message for API | Validation library

The length of 30 characters normally represents that a user of the system should type a password no more than 30 characters long. The typed password however when encoded by the system will be of a different length.

The error occurs because the User that you use to map your database entity has a validation constraint of password not exceeding 30 characters. The process to encode your password can modify the original length of the password. Therefore the error occurs.

You should create another class UserDto which would be accessible from the controller and will represent the information that is exchanged between the controller class and the actual user of the system which types the password. This new UserDto can then have this constraint of 30 characters and the database representation with User class can be without such constraint, since in database the password will be stored with a different length.

So you will use UserDto class to validate the input of the user, to the system, and you will use User class to map your database records inside a java object following the structure of the relative table.

By extending FormGroup and calling super with { validator: CustomValidators.passwordMatchValidator } you don't comply with the actual constructor signature of FormGroup which is supposed to take a plain ValidatorFn as second parameter, not embedded in an object like you did.

Try like this:

Gin gonic uses the package github.com/go-playground/validator/v10 to perform binding validation. If the validation fails, the error returned is a validator.ValidationErrors.

This is not mentioned explicitly but here in Model binding and validation it states:

Gin uses go-playground/validator/v10 for validation. Check the full docs on tags usage here.

That links to the go-playground/validator/v10 documentation, where you find the paragraph Validation Functions Return Type error.

You can use the standard errors package to check if the error is that, unwrap it, and access the single fields, which are validator.FieldError. From that, you can construct whatever error message you want.

Given an error model like this:

In mongoose, there is no such method on models called findUserByEmail that's why you are getting that error.

You can use findOne instead. If you insist, I think you should define it yourself in your model.

Thanks to @Eliseo's suggestion I was able to solve the error.

I changed the writeValue function to this:

It sounds like what you are trying to achieve is:

• Watch each FormControl (nested in a FormGroup) to see whether it's value is the same as the value of any other FormControl (nested in a FormGroup) in a given FormArray

At a high level you then have two things you need to achieve:

• Get the value of an individual FormControl and compare it to a list of all of the other values (i.e. as you already have done in your hasDuplicate validator function)
• Assign an error to the individual FormGroups which contain the FormControls which are duplicate

The problem with creating a validator that will sit on the FormArray is that the error you return will be assigned to the FormArray itself and not the individual FormGroups. For example, if hasDuplicate() returns an error , you will have this resulting form structure:

If I am understanding correctly, you only want the second control to be required if there is a specific value in your other control, the dropdown list.

There are a couple ways to do this,

1. You can dynamically add and remove the required validator from that control,

You can create a test FormGroup which consists of 2 password controls and apply validatePasswords validator to this group:

Since both List and Set are Collections and you don't care about specifics of its implementations (you only intend to iterate through elements and validate them) you could simply use implements ConstraintValidator>