Add fetch method to fetch from opts_hash

Add a line to CHANGELOG about fetch method

Add specs for `#fetch`

`#fetch` will pass the method to the option hash.

CHANGELOG.md

@@ -3,6 +3,7 @@
 
 * [#215](https://github.com/ruby-grape/grape-entity/pull/217): Fix: `#delegate_attribute` no longer delegates to methods included with `Kernel` - [@maltoe](https://github.com/maltoe).
 * [#219](https://github.com/ruby-grape/grape-entity/pull/219): Fix: double pass options in serializable_hash - [@sbatykov](https://github.com/sbatykov).
+* [#226](https://github.com/ruby-grape/grape-entity/pull/226): Add fetch method to fetch from opts_hash - [@alanjcfs](https://github.com/alanjcfs).
 * Your contribution here.
 
 0.5.1 (2016-4-4)

lib/grape_entity/options.rb

@@ -15,6 +15,10 @@ def [](key)
         @opts_hash[key]
       end
 
+      def fetch(*args)
+        @opts_hash.fetch(*args)
+      end
+
       def key?(key)
         @opts_hash.key? key
       end

spec/grape_entity/entity_spec.rb

@@ -1729,5 +1729,39 @@ class UserEntity < Grape::Entity
         end
       end
     end
+
+    describe Grape::Entity::Options do
+      module EntitySpec
+        class Crystalline
+          attr_accessor :prop1, :prop2
+
+          def initialize
+            @prop1 = 'value1'
+            @prop2 = 'value2'
+          end
+        end
+
+        class CrystallineEntity < Grape::Entity
+          expose :prop1, if: ->(_, options) { options.fetch(:signal) }
+          expose :prop2, if: ->(_, options) { options.fetch(:beam, 'destructive') == 'destructive' }
+        end
+      end
+
+      context '#fetch' do
+        it 'without passing in a required option raises KeyError' do
+          expect { EntitySpec::CrystallineEntity.represent(EntitySpec::Crystalline.new).as_json }.to raise_error KeyError
+        end
+
+        it 'passing in a required option will expose the values' do
+          crystalline_entity = EntitySpec::CrystallineEntity.represent(EntitySpec::Crystalline.new, signal: true)
+          expect(crystalline_entity.as_json).to eq(prop1: 'value1', prop2: 'value2')
+        end
+
+        it 'with an option that is not default will not expose that value' do
+          crystalline_entity = EntitySpec::CrystallineEntity.represent(EntitySpec::Crystalline.new, signal: true, beam: 'intermittent')
+          expect(crystalline_entity.as_json).to eq(prop1: 'value1')
+        end
+      end
+    end
   end
 end