binding

binding

Request data binding for Martini.

API Reference

Description

Package binding provides several middleware for transforming raw request data into populated structs, validating the input, and handling the errors. Each handler is independent and optional.

Bind

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

1. Deserializes the request data into a struct you supply
2. Performs validation with binding.Validate
3. Bails out with binding.ErrorHandler if there are any errors

Your application (the final handler) will not even see the request if there are any errors.

It reads the Content-Type of the request to know how to deserialize it, or if the Content-Type is not specified, it tries different deserializers until one returns without errors.

Important safety tip: 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.

Form

binding.Form deserializes form data from the request, whether in the query string or as a form-urlencoded payload, and puts the data into a struct you pass in. It then invokes the binding.Validate middleware to perform validation. No error handling is performed, but you can get the errors in your handler by receiving a binding.Errors type.

Json

binding.Json deserializes JSON data in the payload of the request and uses binding.Validate to perform validation. Similar to binding.Form, no error handling is performed, but you can get the errors and handle them yourself.

Validate

binding.Validate receives a populated struct and checks it for errors, first by enforcing the binding:"required" value on struct field tags, then by executing the Validate() method on the struct, if it is a binding.Validator. (See usage below for an example.)

Note: Marking a field as "required" means that you do not allow the zero value for that type (i.e. if you want to allow 0 in an int field, do not make it required).

ErrorHandler

binding.ErrorHandler is a small middleware that simply writes a 400 code to the response and also a JSON payload describing the errors, if any errors have been mapped to the context. It does nothing if there are no errors.

Usage

This is a contrived example to show a few different ways to use the binding package.

package main

import (
   "net/http"
   
   "github.com/codegangsta/martini"
   "github.com/codegangsta/martini-contrib/binding"
 )

type BlogPost struct {
	Title   string    `form:"title" json:"title" binding:"required"`
	Content string    `form:"content" json:"content"`
	Views   int       `form:"views" json:"views"`
	unexported string `form:"-"`  // skip binding of unexported fields
}

// This method implements binding.Validator and is executed by the binding.Validate middleware
func (bp BlogPost) Validate(errors *binding.Errors, req *http.Request) {
	if req.Header.Get("X-Custom-Thing") == "" {
		errors.Overall["x-custom-thing"] = "The X-Custom-Thing header is required"
	}
	if len(bp.Title) < 4 {
		errors.Fields["title"] = "Too short; minimum 4 characters"
	} else if len(bp.Title) > 120 {
		errors.Fields["title"] = "Too long; maximum 120 characters"
	}
	if bp.Views < 0 {
		errors.Fields["views"] = "Views must be at least 0"
	}
}

func main() {
	m := martini.Classic()

	m.Post("/blog", binding.Bind(BlogPost{}), func(blogpost BlogPost) string {
		// This function won't execute if there were errors
		return blogpost.Title
	})

	m.Get("/blog", binding.Form(BlogPost{}), binding.ErrorHandler, func(blogpost BlogPost) string {
		// This function won't execute if there were errors
		return blogpost.Title
	})

	m.Get("/blog", binding.Form(BlogPost{}), func(blogpost BlogPost, err binding.Errors, resp http.ResponseWriter) string {
		// This function WILL execute if there are errors because binding.Form doesn't handle errors
		if err.Count() > 0 {
			resp.WriteHeader(http.StatusBadRequest)
		}
		return blogpost.Title
	})

	m.Post("/blog", binding.Json(BlogPost{}), myOwnErrorHandler, func(blogpost BlogPost) string {
		// By this point, I assume that my own middleware took care of any errors
		return blogpost.Title
	})

	m.Run()
}

Authors

• Matthew Holt
• Michael Whatcott
• Jeremy Saenz