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Instance Public methods
as_json(options = nil)

Returns a hash representing the model. Some configuration can be passed through options.

The option include_root_in_json controls the top-level behavior of as_json. If true (the default) as_json will emit a single root node named after the object’s type. For example:

user = User.find(1)
user.as_json
# => { "user": {"id": 1, "name": "Konata Izumi", "age": 16,
                "created_at": "2006/08/01", "awesome": true} }

ActiveRecord::Base.include_root_in_json = false
user.as_json
# => {"id": 1, "name": "Konata Izumi", "age": 16,
      "created_at": "2006/08/01", "awesome": true}

This behavior can also be achieved by setting the :root option to false as in:

user = User.find(1)
user.as_json(root: false)
# =>  {"id": 1, "name": "Konata Izumi", "age": 16,
       "created_at": "2006/08/01", "awesome": true}

The remainder of the examples in this section assume include_root_in_json is set to false.

Without any options, the returned Hash will include all the model’s attributes. For example:

user = User.find(1)
user.as_json
# => {"id": 1, "name": "Konata Izumi", "age": 16,
      "created_at": "2006/08/01", "awesome": true}

The :only and :except options can be used to limit the attributes included, and work similar to the attributes method. For example:

user.as_json(:only => [ :id, :name ])
# => {"id": 1, "name": "Konata Izumi"}

user.as_json(:except => [ :id, :created_at, :age ])
# => {"name": "Konata Izumi", "awesome": true}

To include the result of some method calls on the model use :methods:

user.as_json(:methods => :permalink)
# => {"id": 1, "name": "Konata Izumi", "age": 16,
      "created_at": "2006/08/01", "awesome": true,
      "permalink": "1-konata-izumi"}

To include associations use :include:

user.as_json(:include => :posts)
# => {"id": 1, "name": "Konata Izumi", "age": 16,
      "created_at": "2006/08/01", "awesome": true,
      "posts": [{"id": 1, "author_id": 1, "title": "Welcome to the weblog"},
                {"id": 2, author_id: 1, "title": "So I was thinking"}]}

Second level and higher order associations work as well:

user.as_json(:include => { :posts => {
                               :include => { :comments => {
                                             :only => :body } },
                               :only => :title } })
# => {"id": 1, "name": "Konata Izumi", "age": 16,
      "created_at": "2006/08/01", "awesome": true,
      "posts": [{"comments": [{"body": "1st post!"}, {"body": "Second!"}],
                 "title": "Welcome to the weblog"},
                {"comments": [{"body": "Don't think too hard"}],
                 "title": "So I was thinking"}]}

Source: show | on GitHub 

# File activemodel/lib/active_model/serializers/json.rb, line 89
def as_json(options = nil)
  root = include_root_in_json
  root = options[:root] if options.try(:key?, :root)
  if root
    root = self.class.model_name.element if root == true
    { root => serializable_hash(options) }
  else
    serializable_hash(options)
  end
end
from_json(json, include_root=include_root_in_json)

Source: show | on GitHub 

# File activemodel/lib/active_model/serializers/json.rb, line 100
def from_json(json, include_root=include_root_in_json)
  hash = ActiveSupport::JSON.decode(json)
  hash = hash.values.first if include_root
  self.attributes = hash
  self
end