sheriff

go get github.com/liip/sheriff

Package sheriff marshals structs conditionally based on tags on the fields.

A typical use is an API which marshals structs into JSON and maintains different API versions. Using sheriff, struct fields can be annotated with API version and group tags.

By invoking sheriff with specific options, those tags determine whether a field will be added to the output map or not. It can then be marshalled using "encoding/json".

NOTE: This package is tested only on Go 1.7+, it might work on Go 1.6 too, but no support is given.

Implemented tags

Groups

Groups can be used for limiting the output based on freely defined parameters. For example: restrict marshalling the email address of a user to the user itself by just adding the group personal if the user fetches his profile. Multiple groups can be separated by comma.

Example:

type GroupsExample struct {
    Username      string `json:"username" groups:"api"`
    Email         string `json:"email" groups:"personal"`
    SomethingElse string `json:"something_else" groups:"api,personal"`
}

Anonymous fields

Tags added to a struct’s anonymous field propagates to the inner-fields if no other tags are specified.

Example:

type UserInfo struct {
    UserPrivateInfo `groups:"private"`
    UserPublicInfo  `groups:"public"`
}
type UserPrivateInfo struct {
    Age string
}
type UserPublicInfo struct {
    ID    string
    Email string
}

Since

Since specifies the version since that field is available. It's inclusive and SemVer compatible using github.com/hashicorp/go-version. If you specify version 2 in a tag, this version will be output in case you specify version >=2.0.0 as the API version.

Example:

type SinceExample struct {
    Username string `json:"username" since:"2.1.0"`
    Email    string `json:"email" since:"2"`
}

Until

Until specifies the version until that field is available. It's the opposite of since, inclusive and SemVer compatible using github.com/hashicorp/go-version. If you specify version 2 in a tag, this version will be output in case you specify version <=2.0.0 as the API version.

Example:

type UntilExample struct {
    Username string `json:"username" until:"2.1.0"`
    Email    string `json:"email" until:"2"`
}

Example

package main

import (
	"encoding/json"
	"fmt"
	"log"

	"github.com/hashicorp/go-version"
	"github.com/liip/sheriff"
)

type User struct {
	Username string   `json:"username" groups:"api"`
	Email    string   `json:"email" groups:"personal"`
	Name     string   `json:"name" groups:"api"`
	Roles    []string `json:"roles" groups:"api" since:"2"`
}

type UserList []User

func MarshalUsers(version *version.Version, groups []string, users UserList) ([]byte, error) {
	o := &sheriff.Options{
		Groups:     groups,
		ApiVersion: version,
	}

	data, err := sheriff.Marshal(o, users)
	if err != nil {
		return nil, err
	}

	return json.MarshalIndent(data, "", "  ")
}

func main() {
	users := UserList{
		User{
			Username: "alice",
			Email:    "[email protected]",
			Name:     "Alice",
			Roles:    []string{"user", "admin"},
		},
		User{
			Username: "bob",
			Email:    "[email protected]",
			Name:     "Bob",
			Roles:    []string{"user"},
		},
	}

	v1, err := version.NewVersion("1.0.0")
	if err != nil {
		log.Panic(err)
	}
	v2, err := version.NewVersion("2.0.0")

	output, err := MarshalUsers(v1, []string{"api"}, users)
	if err != nil {
		log.Panic(err)
	}
	fmt.Println("Version 1 output:")
	fmt.Printf("%s\n\n", output)

	output, err = MarshalUsers(v2, []string{"api"}, users)
	if err != nil {
		log.Panic(err)
	}
	fmt.Println("Version 2 output:")
	fmt.Printf("%s\n\n", output)

	output, err = MarshalUsers(v2, []string{"api", "personal"}, users)
	if err != nil {
		log.Panic(err)
	}
	fmt.Println("Version 2 output with personal group too:")
	fmt.Printf("%s\n\n", output)

}

// Output:
// Version 1 output:
// [
//   {
//     "name": "Alice",
//     "username": "alice"
//   },
//   {
//     "name": "Bob",
//     "username": "bob"
//   }
// ]
//
// Version 2 output:
// [
//   {
//     "name": "Alice",
//     "roles": [
//       "user",
//       "admin"
//     ],
//     "username": "alice"
//   },
//   {
//     "name": "Bob",
//     "roles": [
//       "user"
//     ],
//     "username": "bob"
//   }
// ]
//
// Version 2 output with personal group too:
// [
//   {
//     "email": "[email protected]",
//     "name": "Alice",
//     "roles": [
//       "user",
//       "admin"
//     ],
//     "username": "alice"
//   },
//   {
//     "email": "[email protected]",
//     "name": "Bob",
//     "roles": [
//       "user"
//     ],
//     "username": "bob"
//   }
// ]

Benchmarks

There's a simple benchmark in bench_test.go which compares running sheriff -> JSON versus just marshalling into JSON and runs on every build. Just marshalling JSON itself takes usually between 3 and 5 times less nanoseconds per operation compared to running sheriff and JSON.

Want to make sheriff faster? Please send us your pull request or open an issue discussing a possible improvement 🚀!

Acknowledgements

• This idea and code has been created partially during a Liip hackday.
• Thanks to @basgys for reviews & improvements.

Related projects

mweibel/php-to-go is a code generator translating PHP models (using JMS serializer) to Go structs with sheriff tags. The two projects were initially developed together and just recently php-to-go has been split out and published too.