Data Binding and Validation

Middlware binding provides request data binding and validation for Macaron Instances.


go get


Getting form data from a request

Suppose you have a contact form on your site where at least name and message are required. We'll need a struct to receive the data:

type ContactForm struct {
    Name           string `form:"name" binding:"Required"`
    Email          string `form:"email"`
    Message        string `form:"message" binding:"Required"`
    MailingAddress string `form:"mailing_address"`

Then we simply add our route in Macaron:

m.Post("/contact/submit", binding.Bind(ContactForm{}), func(contact ContactForm) string {
    return fmt.Sprintf("Name: %s\nEmail: %s\nMessage: %s\nMailing Address: %v",
        contact.Name, contact.Email, contact.Message, contact.MailingAddress)

That's it! The binding.Bind function takes care of validating required fields.

By default, if there are any errors (like a required field is empty), binding middleware will return an error to the client and your app won't even see the request. To prevent this behavior, you can use binding.BindIgnErr instead.

Don't try to bind to embedded struct pointers; it won't work. See martini-contrib/binding issue 30 if you want to help with this.

Naming Convention

By default, there is one naming convention for form tag name, which are:

  • Name -> name

  • UnitPrice -> unit_price

For example, previous example can be simplified with following code:

type ContactForm struct {
    Name           string `binding:"Required"`
    Email          string
    Message        string `binding:"Required"`
    MailingAddress string

Clean and neat, isn't it?

If you want to custom your app naming convention, you can use binding.SetNameMapper function, which accepts a function that is type of binding.NameMapper.

Getting JSON data from a request

To get data from JSON payloads, simply use the json: struct tags instead of form:.

Use JSON-to-Go to correctly convert JSON to a Go type definition. It's useful if you're new to this or the structure is large/complex.

Binding to interfaces

If you'd like to bind the data to an interface rather than to a concrete struct, you can specify the interface and use it like this:

m.Post("/contact/submit", binding.Bind(ContactForm{}, (*MyInterface)(nil)), func(contact MyInterface) {
    // ... your struct became an interface!

Description of Handlers

Each of these middleware handlers are independent and optional, though be aware that some handlers invoke other ones.


binding.Bind is a convenient wrapper over the other handlers in this package. It does the following boilerplate for you:

  1. Deserializes request data into a struct

  2. Performs validation with binding.Validate

  3. If your struct doesn't implement binding.ErrorHandler, then default error handling will be applied. Otherwise, calls ErrorHandler.Error method to perform custom error handling.


  • Your application (the final handler) will not even see the request if there are any errors when default error handling is applied.

  • Header Content-Type will be used to know how to deserialize the requests.

Don't attempt to bind a pointer to a struct. This will cause a panic to prevent a race condition where every request would be pointing to the same struct.


binding.Form deserializes form data from the request, whether in the query string or as a form-urlencoded payload. It only does these things:

  1. Deserializes request data into a struct

  2. Performs validation with binding.Validate

Note that it does not handle errors. You may receive a binding.Errors into your own handler if you want to handle errors.

MultipartForm and File Uploads

Like binding.Form, binding.MultipartForm deserializes form data from a request into the struct you pass in. Additionally, this will deserialize a POST request that has a form of enctype="multipart/form-data". If the bound struct contains a field of type *multipart.FileHeader (or []*multipart.FileHeader), you also can read any uploaded files that were part of the form.

This handler does the following:

  1. Deserializes request data into a struct

  2. Performs validation with binding.Validate

Again, like binding.Form, no error handling is performed, but you can get the errors in your handler by receiving a binding.Errors type.


type UploadForm struct {
    Title      string                `form:"title"`
    TextUpload *multipart.FileHeader `form:"txtUpload"`

func main() {
    m := macaron.Classic()
    m.Post("/", binding.MultipartForm(UploadForm{}), uploadHandler(uf UploadForm) string {
        file, err := uf.TextUpload.Open()
        // ... you can now read the uploaded file


binding.Json deserializes JSON data in the payload of the request. It does the following things:

  1. Deserializes request data into a struct

  2. Performs validation with binding.Validate

Similar to binding.Form, no error handling is performed, but you can get the errors and handle them yourself.


binding.Validate receives a populated struct and checks it for errors with basic rules. It will execute the Validator.Validate() method on the struct, if it is a binding.Validator.

Validation Rules

There are some builtin validation rules. To use them, the tag format is binding:"<Name>".




Omit rest of validations if value is empty


Must be non-zero value


Must be alpha characters or numerics or -_


Must be alpha characters or numerics, -_ or .


Fixed length


Minimum length


Maximum length


Value range(inclusive)


Must be E-mail address


Must be HTTP/HTTPS URL address


Must be one of element in array


Must not be one of element in array


Must contain


Must not contain


Set default value when field is zero-value(cannot use this when bind with interface wrapper)

To combine multiple rules: binding:"Required;MinSize(10)".

Customize Operations

Custom Validation

If you want additional validation beyond just checking required fields, your struct can implement the binding.Validator interface like so:

func (cf ContactForm) Validate(ctx *macaron.Context, errs binding.Errors) binding.Errors {
    if strings.Contains(cf.Message, "Go needs generics") {
        errs = append(errors, binding.Error{
            FieldNames:     []string{"message"},
            Classification: "ComplaintError",
            Message:        "Go has generics. They're called interfaces.",
    return errs

Now, any contact form submissions with "Go needs generics" in the message will return an error explaining your folly.

Custom Validation Rules

If you need to more validation rules that are applied automatically for you, you can add custom rules by function binding.AddParamRule, it accepts type binding.ParamRule as argument.

Suppose you want to limit minimum value:

    IsMatch: func(rule string) bool {
        return strings.HasPrefix(rule, "Min(")
    IsValid: func(errs binding.Errors, rule, name string, v interface{}) (bool, binding.Errors) {
        num, ok := v.(int)
        if !ok {
            return false, errs
        min, _ := strconv.Atoi(rule[4 : len(rule)-1])
        if num < min {
            errs.Add([]string{name}, "MinimumValue", "Value is too small")
            return false, errs
        return true, errs

If your rule is simple, you can also use binding.AddRule, it accepts type binding.Rule:

    IsMatch: func(rule string) bool {
        return rule == "String"
    IsValid: func(errs binding.Errors, name string, v interface{}) (bool, binding.Errors) {
        _, ok := v.(string)
        return ok, errs

Custom validation rules are applied after builtin rules.

Custom Error Handler

If you want to avoid default error handle process but still want binding middleware calls handle function for you, your struct can implement the binding.ErrorHandler interface like so:

func (cf ContactForm) Error(ctx *macaron.Context, errs binding.Errors) {
    // Custom process to handle error.

This operation happens after your custom validation.

Last updated