jsonapi

A serializer/deserializer for json payloads that comply to the JSON API - jsonapi.org spec in go.

Installation

go get -u github.com/google/jsonapi

Or, see Alternative Installation.

Background

You are working in your Go web application and you have a struct that is organized similarly to your database schema. You need to send and receive json payloads that adhere to the JSON API spec. Once you realize that your json needed to take on this special form, you go down the path of creating more structs to be able to serialize and deserialize JSON API payloads. Then there are more models required with this additional structure. Ugh! With JSON API, you can keep your model structs as is and use StructTags to indicate to JSON API how you want your response built or your request deserialized. What about your relationships? JSON API supports relationships out of the box and will even put them in your response into an included side-loaded slice--that contains associated records.

Introduction

JSON API uses StructField tags to annotate the structs fields that you already have and use in your app and then reads and writes JSON API output based on the instructions you give the library in your JSON API tags. Let's take an example. In your app, you most likely have structs that look similar to these:

type Blog struct {
	ID            int       `json:"id"`
	Title         string    `json:"title"`
	Posts         []*Post   `json:"posts"`
	CurrentPost   *Post     `json:"current_post"`
	CurrentPostId int       `json:"current_post_id"`
	CreatedAt     time.Time `json:"created_at"`
	ViewCount     int       `json:"view_count"`
}

type Post struct {
	ID       int        `json:"id"`
	BlogID   int        `json:"blog_id"`
	Title    string     `json:"title"`
	Body     string     `json:"body"`
	Comments []*Comment `json:"comments"`
}

type Comment struct {
	Id     int    `json:"id"`
	PostID int    `json:"post_id"`
	Body   string `json:"body"`
	Likes  uint   `json:"likes_count,omitempty"`
}

These structs may or may not resemble the layout of your database. But these are the ones that you want to use right? You wouldn't want to use structs like those that JSON API sends because it is difficult to get at all of your data easily.

Example App

examples/app.go

This runnable file demonstrates the implementation of a create, a show, and a list http.Handler. It outputs some example requests and responses as well as serialized examples of the source/target structs to json. That is to say, I show you that the library has successfully taken your JSON API request and turned it into your struct types.

To run,

• Make sure you have go installed
• Create the following directories or similar: ~/go
• Set GOPATH to PWD in your shell session, export GOPATH=$PWD
• go get github.com/google/jsonapi. (Append -u after get if you are updating.)
• go run $GOPATH/src/github.com/google/jsonapi/examples/app.go or cd $GOPATH/src/github.com/google/jsonapi/examples && go run app.go

jsonapi Tag Reference

Example

The jsonapi StructTags tells this library how to marshal and unmarshal your structs into JSON API payloads and your JSON API payloads to structs, respectively. Then Use JSON API's Marshal and Unmarshal methods to construct and read your responses and replies. Here's an example of the structs above using JSON API tags:

type Blog struct {
	ID            int       `jsonapi:"primary,blogs"`
	Title         string    `jsonapi:"attr,title"`
	Posts         []*Post   `jsonapi:"relation,posts"`
	CurrentPost   *Post     `jsonapi:"relation,current_post"`
	CurrentPostID int       `jsonapi:"attr,current_post_id"`
	CreatedAt     time.Time `jsonapi:"attr,created_at"`
	ViewCount     int       `jsonapi:"attr,view_count"`
}

type Post struct {
	ID       int        `jsonapi:"primary,posts"`
	BlogID   int        `jsonapi:"attr,blog_id"`
	Title    string     `jsonapi:"attr,title"`
	Body     string     `jsonapi:"attr,body"`
	Comments []*Comment `jsonapi:"relation,comments"`
}

type Comment struct {
	ID     int    `jsonapi:"primary,comments"`
	PostID int    `jsonapi:"attr,post_id"`
	Body   string `jsonapi:"attr,body"`
	Likes  uint   `jsonapi:"attr,likes-count,omitempty"`
}

Permitted Tag Values

primary

`jsonapi:"primary,<type field output>"`

This indicates this is the primary key field for this struct type. Tag value arguments are comma separated. The first argument must be, primary, and the second must be the name that should appear in the type* field for all data objects that represent this type of model.

* According the JSON API spec, the plural record types are shown in the examples, but not required.

attr

`jsonapi:"attr,<key name in attributes hash>,<optional: omitempty>"`

These fields' values will end up in the attributeshash for a record. The first argument must be, attr, and the second should be the name for the key to display in the attributes hash for that record. The optional third argument is omitempty - if it is present the field will not be present in the "attributes" if the field's value is equivalent to the field types empty value (ie if the count field is of type int, omitempty will omit the field when count has a value of 0). Lastly, the spec indicates that attributes key names should be dasherized for multiple word field names.

relation

`jsonapi:"relation,<key name in relationships hash>,<optional: omitempty>"`

Relations are struct fields that represent a one-to-one or one-to-many relationship with other structs. JSON API will traverse the graph of relationships and marshal or unmarshal records. The first argument must be, relation, and the second should be the name of the relationship, used as the key in the relationships hash for the record. The optional third argument is omitempty - if present will prevent non existent to-one and to-many from being serialized.

Methods Reference

All Marshal and Unmarshal methods expect pointers to struct instance or slices of the same contained with the interface{}s

Now you have your structs prepared to be seralized or materialized, What about the rest?

Create Record Example

You can Unmarshal a JSON API payload using jsonapi.UnmarshalPayload. It reads from an io.Reader containing a JSON API payload for one record (but can have related records). Then, it materializes a struct that you created and passed in (using new or &). Again, the method supports single records only, at the top level, in request payloads at the moment. Bulk creates and updates are not supported yet.

After saving your record, you can use, MarshalOnePayload, to write the JSON API response to an io.Writer.

UnmarshalPayload

UnmarshalPayload(in io.Reader, model interface{})

Visit godoc

MarshalOnePayload

MarshalOnePayload(w io.Writer, model interface{}) error

Visit godoc

Writes a JSON API response, with related records sideloaded, into an included array. This method encodes a response for a single record only. If you want to serialize many records, see, MarshalManyPayload.

Handler Example Code

func CreateBlog(w http.ResponseWriter, r *http.Request) {
	blog := new(Blog)

	if err := jsonapi.UnmarshalPayload(r.Body, blog); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	// ...save your blog...

	w.WriteHeader(http.StatusCreated)
	w.Header().Set("Content-Type", jsonapi.MediaType)

	if err := jsonapi.MarshalOnePayload(w, blog); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}

List Records Example

MarshalManyPayload

MarshalManyPayload(w io.Writer, models []interface{}) error

Visit godoc

Takes an io.Writer and an slice of interface{}. Note, if you have a type safe array of your structs, like,

var blogs []*Blog

you will need to iterate over the slice of Blog pointers and append them to an interface array, like,

blogInterface := make([]interface{}, len(blogs))

for i, blog := range blogs {
  blogInterface[i] = blog
}

Alternatively, you can insert your Blogs into a slice of interface{} the first time. For example when you fetch the Blogs from the db append them to an []interface{} rather than a []*Blog. So your method signature to reach into your data store may look something like this,

func FetchBlogs() ([]interface{}, error)

Handler Example Code

func ListBlogs(w http.ResponseWriter, r *http.Request) {
	// ...fetch your blogs, filter, offset, limit, etc...

  // but, for now
	blogs := testBlogsForList()

	w.WriteHeader(http.StatusOK)
	w.Header().Set("Content-Type", jsonapi.MediaType)
	if err := jsonapi.MarshalManyPayload(w, blogs); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}

Create Records Example

UnmarshalManyPayload

UnmarshalManyPayload(in io.Reader, t reflect.Type) ([]interface{}, error)

Visit godoc

Takes an io.Reader and a reflect.Type representing the uniform type contained within the "data" JSON API member.

Handler Example Code

func CreateBlogs(w http.ResponseWriter, r *http.Request) {
	// ...create many blogs at once

	blogs, err := UnmarshalManyPayload(r.Body, reflect.TypeOf(new(Blog)))
	if err != nil {
		t.Fatal(err)
	}

	for _, blog := range blogs {
		b, ok := blog.(*Blog)
		// ...save each of your blogs
	}

	w.WriteHeader(http.StatusCreated)
	w.Header().Set("Content-Type", jsonapi.MediaType)
	if err := jsonapi.MarshalManyPayload(w, blogs); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}

Links

If you need to include link objects along with response data, implement the Linkable interface for document-links, and RelationshipLinkable for relationship links:

func (post Post) JSONAPILinks() *Links {
	return &Links{
		"self": "href": fmt.Sprintf("https://example.com/posts/%d", post.ID),
		"comments": Link{
			Href: fmt.Sprintf("https://example.com/api/blogs/%d/comments", post.ID),
			Meta: map[string]interface{}{
				"counts": map[string]uint{
					"likes":    4,
				},
			},
		},
	}
}

// Invoked for each relationship defined on the Post struct when marshaled
func (post Post) JSONAPIRelationshipLinks(relation string) *Links {
	if relation == "comments" {
		return &Links{
			"related": fmt.Sprintf("https://example.com/posts/%d/comments", post.ID),
		}
	}
	return nil
}

Errors

This package also implements support for JSON API compatible errors payloads using the following types.

MarshalErrors

MarshalErrors(w io.Writer, errs []*ErrorObject) error

Writes a JSON API response using the given []error.

ErrorsPayload

type ErrorsPayload struct {
	Errors []*ErrorObject `json:"errors"`
}

ErrorsPayload is a serializer struct for representing a valid JSON API errors payload.

ErrorObject

type ErrorObject struct { ... }

// Error implements the `Error` interface.
func (e *ErrorObject) Error() string {
	return fmt.Sprintf("Error: %s %s\n", e.Title, e.Detail)
}

ErrorObject is an Error implementation as well as an implementation of the JSON API error object.

The main idea behind this struct is that you can use it directly in your code as an error type and pass it directly to MarshalErrors to get a valid JSON API errors payload.

Errors Example Code

// An error has come up in your code, so set an appropriate status, and serialize the error.
if err := validate(&myStructToValidate); err != nil {
	context.SetStatusCode(http.StatusBadRequest) // Or however you need to set a status.
	jsonapi.MarshalErrors(w, []*ErrorObject{{
		Title: "Validation Error",
		Detail: "Given request body was invalid.",
		Status: "400",
		Meta: map[string]interface{}{"field": "some_field", "error": "bad type", "expected": "string", "received": "float64"},
	}})
	return
}

Testing

MarshalOnePayloadEmbedded

MarshalOnePayloadEmbedded(w io.Writer, model interface{}) error

Visit godoc

This method is not strictly meant to for use in implementation code, although feel free. It was mainly created for use in tests; in most cases, your request payloads for create will be embedded rather than sideloaded for related records. This method will serialize a single struct pointer into an embedded json response. In other words, there will be no, included, array in the json; all relationships will be serialized inline with the data.

However, in tests, you may want to construct payloads to post to create methods that are embedded to most closely model the payloads that will be produced by the client. This method aims to enable that.

Example

out := bytes.NewBuffer(nil)

// testModel returns a pointer to a Blog
jsonapi.MarshalOnePayloadEmbedded(out, testModel())

h := new(BlogsHandler)

w := httptest.NewRecorder()
r, _ := http.NewRequest(http.MethodPost, "/blogs", out)

h.CreateBlog(w, r)

blog := new(Blog)
jsonapi.UnmarshalPayload(w.Body, blog)

// ... assert stuff about blog here ...

Alternative Installation

I use git subtrees to manage dependencies rather than go get so that the src is committed to my repo.

git subtree add --squash --prefix=src/github.com/google/jsonapi https://github.com/google/jsonapi.git master

To update,

git subtree pull --squash --prefix=src/github.com/google/jsonapi https://github.com/google/jsonapi.git master

This assumes that I have my repo structured with a src dir containing a collection of packages and GOPATH is set to the root folder--containing src.

Contributing

Fork, Change, Pull Request with tests.