How Can I Customize Error Messages from Go Gin's Struct Tag Validation?

Customizing Error Messages for Struct Tag Validation in Go Gin

Gin, a popular web framework for Go, utilizes go-playground/validator/v10 for struct validation. When validation fails, it typically returns verbose error messages. This article explores how to customize these messages to provide a more user-friendly experience.

Understanding the Error Type

The error returned by Gin's validation is a validator.ValidationErrors. This error type contains a collection of validator.FieldErrors, each representing an invalid field and its validation tag.

Customizing Error Messages

To customize the error messages, we can convert the validator.ValidationErrors to error using the errors.As function. Once converted, we can access the individual validator.FieldErrors and construct custom error messages based on the validation tag used.

Creating a Custom Error Model

First, we define a custom error model, such as:

type ApiError struct {
    Field string
    Msg   string
}

Customizing Error Messages for Validation Tags

Next, we define a helper function to map validation tags to custom error messages:

func msgForTag(tag string) string {
    switch tag {
    case "required":
        return "This field is required"
    case "email":
        return "Invalid email"
    }
    return ""
}

Binding and Handling Validation Errors

In our handler function, we can bind the request to our struct and check for validation errors:

var u User
err := c.BindQuery(&u)
if err != nil {
    var ve validator.ValidationErrors
    if errors.As(err, &ve) {
        out := make([]ApiError, len(ve))
        for i, fe := range ve {
            out[i] = ApiError{fe.Field(), msgForTag(fe.Tag())}
        }
        c.JSON(http.StatusBadRequest, gin.H{"errors": out})
    }
    return
}

Example Output

Using this custom error handling, we can return a JSON response with the following format:

{
    "errors": [
        {
            "Field": "Number",
            "Msg": "This field is required"
        }
    ]
}

This provides a more user-friendly and informative error response to the user.

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