Allow block for id customization

Allow an ID of object to be customized directly on the serializer by
passing a block to `set_id` as opposed to only through a model property.

We already allow for attributes that do not have a model property of the
same name to be customized directly on the serializer using a block.

This customization can be useful in situation in which you have
different classes being serialized using the same serializer. For
example, if we have `HorrorMovie`, `ComedyMovie` and `DramaMovie` using
the same `MovieSerializer`, we can unify their IDs using

```
class MovieSerializer
  include FastJsonapi::ObjectSerializer

  attributes :name, :year
  set_id do |record|
    "#{record.name.downcase}-#{record.id}"
  end
```

which is preferable to creating a `#serialized_id` method in every model
that will use `MovieSerializer` to encapsulate the customization.

Closes #315

lib/fast_jsonapi/object_serializer.rb

@@ -167,8 +167,8 @@ module FastJsonapi
         self.record_type = run_key_transform(type_name)
       end
 
-      def set_id(id_name)
-        self.record_id = id_name
+      def set_id(id_name = nil, &block)
+        self.record_id = block || id_name
       end
 
       def cache_options(cache_options)

lib/fast_jsonapi/serialization_core.rb

@@ -86,9 +86,10 @@ module FastJsonapi
       end
 
       def id_from_record(record)
-         return record.send(record_id) if record_id
-         raise MandatoryField, 'id is a mandatory field in the jsonapi spec' unless record.respond_to?(:id)
-         record.id
+        return record_id.call(record) if record_id.is_a?(Proc)
+        return record.send(record_id) if record_id
+        raise MandatoryField, 'id is a mandatory field in the jsonapi spec' unless record.respond_to?(:id)
+        record.id
       end
 
       # Override #to_json for alternative implementation

spec/lib/object_serializer_class_methods_spec.rb

@@ -195,28 +195,57 @@ describe FastJsonapi::ObjectSerializer do
   describe '#set_id' do
     subject(:serializable_hash) { MovieSerializer.new(resource).serializable_hash }
 
-    before do
-      MovieSerializer.set_id :owner_id
-    end
+    context 'method name' do
+      before do
+        MovieSerializer.set_id :owner_id
+      end
 
-    after do
-      MovieSerializer.set_id nil
-    end
+      after do
+        MovieSerializer.set_id nil
+      end
 
-    context 'when one record is given' do
-      let(:resource) { movie }
+      context 'when one record is given' do
+        let(:resource) { movie }
 
-      it 'returns correct hash which id equals owner_id' do
-        expect(serializable_hash[:data][:id].to_i).to eq movie.owner_id
+        it 'returns correct hash which id equals owner_id' do
+          expect(serializable_hash[:data][:id].to_i).to eq movie.owner_id
+        end
+      end
+
+      context 'when an array of records is given' do
+        let(:resource) { [movie, movie] }
+
+        it 'returns correct hash which id equals owner_id' do
+          expect(serializable_hash[:data][0][:id].to_i).to eq movie.owner_id
+          expect(serializable_hash[:data][1][:id].to_i).to eq movie.owner_id
+        end
       end
     end
 
-    context 'when an array of records is given' do
-      let(:resource) { [movie, movie] }
+    context 'with block' do
+      before do
+        MovieSerializer.set_id { |record| "movie-#{record.owner_id}" }
+      end
 
-      it 'returns correct hash which id equals owner_id' do
-        expect(serializable_hash[:data][0][:id].to_i).to eq movie.owner_id
-        expect(serializable_hash[:data][1][:id].to_i).to eq movie.owner_id
+      after do
+        MovieSerializer.set_id nil
+      end
+
+      context 'when one record is given' do
+        let(:resource) { movie }
+
+        it 'returns correct hash which id equals movie-id' do
+          expect(serializable_hash[:data][:id]).to eq "movie-#{movie.owner_id}"
+        end
+      end
+
+      context 'when an array of records is given' do
+        let(:resource) { [movie, movie] }
+
+        it 'returns correct hash which id equals movie-id' do
+          expect(serializable_hash[:data][0][:id]).to eq "movie-#{movie.owner_id}"
+          expect(serializable_hash[:data][1][:id]).to eq "movie-#{movie.owner_id}"
+        end
       end
     end
   end