Add PoC for resource serializers

app/serializers/base_serializer.rb

+class BaseSerializer
+  def initialize(request = {})
+    @request = EntityRequest.new(request)
+    @opts = { request: @request }
+  end
+
+  def set(opts)
+    @request.merge!(opts)
+    self
+  end
+
+  def represent(resource, opts = {})
+    self.class.entity_class
+      .represent(resource, @opts.reverse_merge(opts))
+  end
+
+  def self.entity(entity_class)
+    @entity_class ||= entity_class
+  end
+
+  def self.entity_class
+    @entity_class
+  end
+end

app/serializers/entity_request.rb

+class EntityRequest
+  # We use EntityRequest object to collect parameters and variables
+  # from the controller. Because options that are being passed to the entity
+  # do appear in each entity object  in the chain, we need a way to pass data
+  # that is present in the controller (see  #20045).
+  #
+  def initialize(parameters)
+    merge!(parameters)
+  end
+
+  def merge!(parameters)
+    parameters.each do |key, value|
+      define_singleton_method(key) { value }
+    end
+  end
+end

app/serializers/environment_entity.rb

+class EnvironmentEntity < Grape::Entity
+  include RequestAwareEntity
+  include Gitlab::Routing.url_helpers
+
+  expose :id
+  expose :name
+  expose :project, with: ProjectEntity
+  expose :last_deployment,
+    as: :deployment,
+    using: API::Entities::Deployment
+
+  expose :environment_path
+
+  def environment_path
+    request.path
+  end
+end

app/serializers/environment_serializer.rb

+class EnvironmentSerializer < BaseSerializer
+  entity EnvironmentEntity
+end

app/serializers/project_entity.rb

+class ProjectEntity < Grape::Entity
+  expose :id
+  expose :name
+
+  expose :test do |project|
+    'something'
+  end
+end

app/serializers/request_aware_entity.rb

+module RequestAwareEntity
+  # We use SerializableRequest class to collect parameters and variables
+  # from the controller. Because options that are being passed to the entity
+  # are appear in each entity in the chain, we need a way to access data
+  # that is present in the controller (see  #20045).
+  #
+  def request
+    options[:request] ||
+      raise(StandardError, 'Request not set!!')
+  end
+end

spec/serializers/entity_request_spec.rb

+require 'spec_helper'
+
+describe EntityRequest do
+  subject do
+    described_class.new(user: 'user', project: 'some project')
+  end
+
+  describe 'methods created' do
+    it 'defines accessible attributes' do
+      expect(subject.user).to eq 'user'
+      expect(subject.project).to eq 'some project'
+    end
+
+    it 'raises error when attribute is not defined' do
+      expect { subject.some_method }.to raise_error NoMethodError
+    end
+  end
+
+  describe '#merge!' do
+    before { subject.merge!(build: 'some build') }
+
+    it 'appends parameters' do
+      expect(subject.build).to eq 'some build'
+    end
+  end
+end

spec/serializers/environment_serializer_spec.rb

+require 'spec_helper'
+
+describe EnvironmentSerializer do
+  let(:serializer) do
+    described_class.new(path: 'some path').represent(resource)
+  end
+
+  context 'when there is a single object provided' do
+    let(:resource) { create(:environment) }
+
+    it 'shows json' do
+      puts serializer.to_json
+    end
+
+    it 'it generates payload for single object' do
+      expect(parsed_json).to be_an_instance_of Hash
+    end
+  end
+
+  context 'when there is a collection of objects provided' do
+    let(:resource) { create_list(:environment, 2) }
+
+    it 'shows json' do
+      puts serializer.to_json
+    end
+
+    it 'generates payload for collection' do
+      expect(parsed_json).to be_an_instance_of Array
+    end
+  end
+
+  def parsed_json
+    JSON.parse(serializer.to_json)
+  end
+end