# Jbuilder

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[travis]: https://travis-ci.org/rails/jbuilder
[gem]: https://rubygems.org/gems/jbuilder
[codeclimate]: https://codeclimate.com/github/rails/jbuilder
[gemnasium]: https://gemnasium.com/rails/jbuilder

Jbuilder gives you a simple DSL for declaring JSON structures that beats
massaging giant hash structures. This is particularly helpful when the
generation process is fraught with conditionals and loops. Here's a simple
example:

``` ruby
# app/views/message/show.json.jbuilder

json.content format_content(@message.content)
json.(@message, :created_at, :updated_at)

json.author do
  json.name @message.creator.name.familiar
  json.email_address @message.creator.email_address_with_name
  json.url url_for(@message.creator, format: :json)
end

if current_user.admin?
  json.visitors calculate_visitors(@message)
end

json.comments @message.comments, :content, :created_at

json.attachments @message.attachments do |attachment|
  json.filename attachment.filename
  json.url url_for(attachment)
end
```

This will build the following structure:

``` javascript
{
  "content": "<p>This is <i>serious</i> monkey business</p>",
  "created_at": "2011-10-29T20:45:28-05:00",
  "updated_at": "2011-10-29T20:45:28-05:00",

  "author": {
    "name": "David H.",
    "email_address": "'David Heinemeier Hansson' <david@heinemeierhansson.com>",
    "url": "http://example.com/users/1-david.json"
  },

  "visitors": 15,

  "comments": [
    { "content": "Hello everyone!", "created_at": "2011-10-29T20:45:28-05:00" },
    { "content": "To you my good sir!", "created_at": "2011-10-29T20:47:28-05:00" }
  ],

  "attachments": [
    { "filename": "forecast.xls", "url": "http://example.com/downloads/forecast.xls" },
    { "filename": "presentation.pdf", "url": "http://example.com/downloads/presentation.pdf" }
  ]
}
```

To define attribute and structure names dynamically, use the `set!` method:

``` ruby
json.set! :author do
  json.set! :name, 'David'
end

# => "author": { "name": "David" }
```

Top level arrays can be handled directly.  Useful for index and other collection actions.

``` ruby
# @comments = @post.comments

json.array! @comments do |comment|
  next if comment.marked_as_spam_by?(current_user)

  json.body comment.body
  json.author do
    json.first_name comment.author.first_name
    json.last_name comment.author.last_name
  end
end

# => [ { "body": "great post...", "author": { "first_name": "Joe", "last_name": "Bloe" }} ]
```

You can also extract attributes from array directly.

``` ruby
# @people = People.all

json.array! @people, :id, :name

# => [ { "id": 1, "name": "David" }, { "id": 2, "name": "Jamie" } ]
```

Jbuilder objects can be directly nested inside each other.  Useful for composing objects.

``` ruby
class Person
  # ... Class Definition ... #
  def to_builder
    Jbuilder.new do |person|
      person.(self, :name, :age)
    end
  end
end

class Company
  # ... Class Definition ... #
  def to_builder
    Jbuilder.new do |company|
      company.name name
      company.president president.to_builder
    end
  end
end

company = Company.new('Doodle Corp', Person.new('John Stobs', 58))
company.to_builder.target!

# => {"name":"Doodle Corp","president":{"name":"John Stobs","age":58}}
```

You can either use Jbuilder stand-alone or directly as an ActionView template
language. When required in Rails, you can create views ala show.json.jbuilder
(the json is already yielded):

``` ruby
# Any helpers available to views are available to the builder
json.content format_content(@message.content)
json.(@message, :created_at, :updated_at)

json.author do
  json.name @message.creator.name.familiar
  json.email_address @message.creator.email_address_with_name
  json.url url_for(@message.creator, format: :json)
end

if current_user.admin?
  json.visitors calculate_visitors(@message)
end
```


You can use partials as well. The following will render the file
`views/comments/_comments.json.jbuilder`, and set a local variable
`comments` with all this message's comments, which you can use inside
the partial.

```ruby
json.partial! 'comments/comments', comments: @message.comments
```

It's also possible to render collections of partials:

```ruby
json.array! @posts, partial: 'posts/post', as: :post

# or

json.partial! 'posts/post', collection: @posts, as: :post

# or

json.partial! partial: 'posts/post', collection: @posts, as: :post

# or

json.comments @post.comments, partial: 'comment/comment', as: :comment
```

You can pass any objects into partial templates with or without `:locals` option.

```ruby
json.partial! 'sub_template', locals: { user: user }

# or

json.partial! 'sub_template', user: user
```


You can explicitly make Jbuilder object return null if you want:

``` ruby
json.extract! @post, :id, :title, :content, :published_at
json.author do
  if @post.anonymous?
    json.null! # or json.nil!
  else
    json.first_name @post.author_first_name
    json.last_name @post.author_last_name
  end
end
```

Fragment caching is supported, it uses `Rails.cache` and works like caching in
HTML templates:

```ruby
json.cache! ['v1', @person], expires_in: 10.minutes do
  json.extract! @person, :name, :age
end
```

You can also conditionally cache a block by using `cache_if!` like this:

```ruby
json.cache_if! !admin?, ['v1', @person], expires_in: 10.minutes do
  json.extract! @person, :name, :age
end
```

If you are rendering fragments for a collection of objects, have a look at
`jbuilder_cache_multi` gem. It uses fetch_multi (>= Rails 4.1) to fetch
multiple keys at once.

Keys can be auto formatted using `key_format!`, this can be used to convert
keynames from the standard ruby_format to camelCase:

``` ruby
json.key_format! camelize: :lower
json.first_name 'David'

# => { "firstName": "David" }
```

You can set this globally with the class method `key_format` (from inside your
environment.rb for example):

``` ruby
Jbuilder.key_format camelize: :lower
```

Faster JSON backends
--------------------

Jbuilder uses MultiJson, which by default will use the JSON gem. That gem is
currently tangled with ActiveSupport's all-Ruby `#to_json` implementation,
which is slow (fixed in Rails >= 4.1). For faster Jbuilder rendering, you can
specify something like the Yajl JSON generator instead. You'll need to include
the `yajl-ruby` gem in your Gemfile and then set the following configuration
for MultiJson:

``` ruby
require 'multi_json'
MultiJson.use :yajl
 ```
