Grape::Entity

Introduction

This gem adds Entity support to API frameworks, such as Grape. Grape's Entity is an API focused facade that sits on top of an object model.

Example

module API
  module Entities
    class Status < Grape::Entity
      format_with(:iso_timestamp) { |dt| dt.iso8601 }

      expose :user_name
      expose :text, documentation: { type: "String", desc: "Status update text." }
      expose :ip, if: { type: :full }
      expose :user_type, :user_id, if: lambda { |status, options| status.user.public? }
      expose :contact_info do
        expose :phone
        expose :address, using: API::Entities::Address
      end
      expose :digest do |status, options|
        Digest::MD5.hexdigest status.txt
      end
      expose :replies, using: API::Entities::Status, as: :responses
      expose :last_reply, using: API::Entities::Status do |status, options|
        status.replies.last
      end

      with_options(format_with: :iso_timestamp) do
        expose :created_at
        expose :updated_at
      end
    end
  end
end

module API
  module Entities
    class StatusDetailed < API::Entities::Status
      expose :internal_id
    end
  end
end

Reusable Responses with Entities

Entities are a reusable means for converting Ruby objects to API responses. Entities can be used to conditionally include fields, nest other entities, and build ever larger responses, using inheritance.

Defining Entities

Entities inherit from Grape::Entity, and define a simple DSL. Exposures can use runtime options to determine which fields should be visible, these options are available to :if, :unless, and :proc.

Basic Exposure

Define a list of fields that will always be exposed.

expose :user_name, :ip

The field lookup takes several steps

first try entity-instance.exposure
next try object.exposure
next try object.fetch(exposure)
last raise an Exception

Exposing with a Presenter

Don't derive your model classes from Grape::Entity, expose them using a presenter.

expose :replies, using: API::Entities::Status, as: :responses

Presenter classes can also be specified in string format, which helps with circular dependencies.

expose :replies, using: "API::Entities::Status", as: :responses

Conditional Exposure

Use :if or :unless to expose fields conditionally.

expose :ip, if: { type: :full }

expose :ip, if: lambda { |instance, options| options[:type] == :full } # exposed if the function evaluates to true
expose :ip, if: :type # exposed if :type is available in the options hash
expose :ip, if: { type: :full } # exposed if options :type has a value of :full

expose :ip, unless: ... # the opposite of :if

Safe Exposure

Don't raise an exception and expose as nil, even if the :x cannot be evaluated.

expose :ip, safe: true

Nested Exposure

Supply a block to define a hash using nested exposures.

expose :contact_info do
  expose :phone
  expose :address, using: API::Entities::Address
end

You can also conditionally expose attributes in nested exposures:

expose :contact_info do
  expose :phone
  expose :address, using: API::Entities::Address
  expose :email, if: lambda { |instance, options| options[:type] == :full }
end

Collection Exposure

Use root(plural, singular = nil) to expose an object or a collection of objects with a root key.

root 'users', 'user'
expose :id, :name, ...

By default every object of a collection is wrapped into an instance of your Entity class. You can override this behavior and wrap the whole collection into one instance of your Entity class.

As example:

 present_collection true, :collection_name  # `collection_name` is optional and defaults to `items`
 expose :collection_name, using: API::Entities::Items

Runtime Exposure

Use a block or a Proc to evaluate exposure at runtime. The supplied block or Proc will be called with two parameters: the represented object and runtime options.

NOTE: A block supplied with no parameters will be evaluated as a nested exposure (see above).

expose :digest do |status, options|
  Digest::MD5.hexdigest status.txt
end

expose :digest, proc: ... # equivalent to a block

You can also define a method on the entity and it will try that before trying on the object the entity wraps.

class ExampleEntity < Grape::Entity
  expose :attr_not_on_wrapped_object
  # ...
private

  def attr_not_on_wrapped_object
    42
  end
end

You have always access to the presented instance with object

class ExampleEntity < Grape::Entity
  expose :formatted_value
  # ...
private

  def formatted_value
    "+ X #{object.value}"
  end
end

Unexpose

To undefine an exposed field, use the .unexpose method. Useful for modifying inherited entities.

class UserData < Grape::Entity
  expose :name
  expose :address1
  expose :address2
  expose :address_state
  expose :address_city
  expose :email
  expose :phone
end

class MailingAddress < UserData
  unexpose :email
  unexpose :phone
end

Returning only the fields you want

After exposing the desired attributes, you can choose which one you need when representing some object or collection by using the only: and except: options. See the example:

class UserEntity
  expose :id
  expose :name
  expose :email
end

class Entity
  expose :id
  expose :title
  expose :user, using: UserEntity
end

data = Entity.represent(model, only: [:title, { user: [:name, :email] }])
data.as_json

This will return something like this:

{
  title: 'grape-entity is awesome!',
  user: {
    name: 'John Applet',
    email: 'john@example.com'
  }
}

Instead of returning all the exposed attributes.

The same result can be achieved with the following exposure:

data = Entity.represent(model, except: [:id, { user: [:id] }])
data.as_json

Aliases

Expose under a different name with :as.

expose :replies, using: API::Entities::Status, as: :responses

Format Before Exposing

Apply a formatter before exposing a value.

format_with(:iso_timestamp) { |dt| dt.iso8601 }
with_options(format_with: :iso_timestamp) do
  expose :created_at
  expose :updated_at
end

Documentation

Expose documentation with the field. Gets bubbled up when used with Grape and various API documentation systems.

expose :text, documentation: { type: "String", desc: "Status update text." }

Options Hash

The option keys :version and :collection are always defined. The :version key is defined as api.version. The :collection key is boolean, and defined as true if the object presented is an array. The options also contain the runtime environment in :env, which includes request parameters in options[:env]['grape.request.params'].

Any additional options defined on the entity exposure are included as is. In the following example user is set to the value of current_user.

class Status < Grape::Entity
  expose :user, if: lambda { |instance, options| options[:user] } do |instance, options|
    # examine available environment keys with `p options[:env].keys`
    options[:user]
  end
end

present s, with: Status, user: current_user

Passing Additional Option To Nested Exposure

There are sometimes that you want to pass additional option or parameter to nested exposure. Assume that you need to expose an address for a contact info, but it has both two different format: full and simple. You can pass an additional full_format option to specify that if the nested entity should render address in :full format.

# api/contact.rb
expose :contact_info do
  expose :phone
  expose :address do |instance, options|
    # use `#merge` to extend options and then pass the new version of options to the nested entity
    API::Entities::Address.represent instance.address, options.merge(full_format: instance.need_full_format?)
  end
  expose :email, if: lambda { |instance, options| options[:type] == :full }
end

# api/address.rb
expose :state, if: lambda {|instance, options| !!options[:full_format]}      # the new option could be retrieved in options hash for conditional exposure
expose :city, if: lambda {|instance, options| !!options[:full_format]}
expose :stree do |instance, options|
  # the new option could be retrieved in options hash for runtime exposure
  !!options[:full_format] ? instance.full_street_name : instance.simple_street_name
end

Notice: In the above code, you should pay attention to Safe Exposure yourself, for example, instance.address might be nil, in this situation, it is better to expose it as nil directly.

Attribute Path Tracking

Sometimes, especially when there are nested attributes, you might want to know which attribute it is being exposed. For example, some APIs allow user provide a parameter to control which fields will be included in (or excluded from) the response.

Grape entity can track the path of each attribute, then you could access it during condition check or runtime exposure, via options[:attr_path].

Attribute path is an array. The last item of this array is the name (alias) of current attribute. And if the attribute is nested, the former items are names (aliases) of its ancestor attributes.

Here is an example shows what the attribute path will be.

class Status < Grape::Entity
  expose :user  # path is [:user]
  expose :foo, as: :bar  # path is [:bar]
  expose :a do
    expose :b, as: :xx do
      expose :c  # path is [:a, :xx, :c]
    end
  end
end

Using the Exposure DSL

Grape ships with a DSL to easily define entities within the context of an existing class:

class Status
  include Grape::Entity::DSL

  entity :text, :user_id do
    expose :detailed, if: :conditional
  end
end

The above will automatically create a Status::Entity class and define properties on it according to the same rules as above. If you only want to define simple exposures you don't have to supply a block and can instead simply supply a list of comma-separated symbols.

Using Entities

With Grape, once an entity is defined, it can be used within endpoints, by calling present. The present method accepts two arguments, the object to be presented and the options associated with it. The options hash must always include :with, which defines the entity to expose. If the entity includes documentation it can be included in an endpoint's description.

module API
  class Statuses < Grape::API
    version 'v1'

    desc 'Statuses.', {
      params: API::Entities::Status.documentation
    }
    get '/statuses' do
      statuses = Status.all
      type = current_user.admin? ? :full : :default
      present statuses, with: API::Entities::Status, type: type
    end
  end
end

Entity Organization

In addition to separately organizing entities, it may be useful to put them as namespaced classes underneath the model they represent.

class Status
  def entity
    Entity.new(self)
  end

  class Entity < Grape::Entity
    expose :text, :user_id
  end
end

If you organize your entities this way, Grape will automatically detect the Entity class and use it to present your models. In this example, if you added present User.new to your endpoint, Grape would automatically detect that there is a Status::Entity class and use that as the representative entity. This can still be overridden by using the :with option or an explicit represents call.

Caveats

Entities with duplicate exposure names and conditions will silently overwrite one another. In the following example, when object.check equals "foo", only field_a will be exposed. However, when object.check equals "bar" both field_b and foo will be exposed.

module API
  module Entities
    class Status < Grape::Entity
      expose :field_a, :foo, if: lambda { |object, options| object.check == "foo" }
      expose :field_b, :foo, if: lambda { |object, options| object.check == "bar" }
    end
  end
end

This can be problematic, when you have mixed collections. Using respond_to? is safer.

module API
  module Entities
    class Status < Grape::Entity
      expose :field_a, if: lambda { |object, options| object.check == "foo" }
      expose :field_b, if: lambda { |object, options| object.check == "bar" }
      expose :foo, if: lambda { |object, options| object.respond_to?(:foo) }
    end
  end
end

Also note that an ArgumentError is raised when unknown options are passed to either expose or with_options.

Installation

Add this line to your application's Gemfile:

gem 'grape-entity'

And then execute:

$ bundle

Or install it yourself as:

$ gem install grape-entity

Testing with Entities

Test API request/response as usual.

Also see Grape Entity Matchers.

Project Resources

Need help? Grape Google Group

Contributing

See CONTRIBUTING.md.

License

MIT License. See LICENSE for details.

tyre/grape-entity