Add optional meta field to relationships (#99) (#100)

* Add optional meta field to relationships (#99)

* Fix Rubocop's warnings.

* Minor syntax corrections.

README.md

@@ -317,6 +317,23 @@ class MovieSerializer
 end
 ```
 
+#### Meta on a Relationship
+
+You can specify [relationship meta](http://jsonapi.org/format/#document-resource-object-relationships) by using the `meta:` option on the serializer. Relationship meta in JSON API is useful if you wish to provide non-standard meta-information about the relationship.
+
+Meta can be defined either by passing a static hash or by using Proc to the `meta` key. In the latter case, the record and any params passed to the serializer are available inside the Proc as the first and second parameters, respectively.
+
+
+```ruby
+class MovieSerializer
+  include JSONAPI::Serializer
+
+  has_many :actors, meta: Proc.new do |movie_record, params|
+    { count: movie_record.actors.length }
+  end
+end
+```
+
 ### Compound Document
 
 Support for top-level and nested included associations through `options[:include]`.

lib/fast_jsonapi/object_serializer.rb

@@ -283,6 +283,7 @@ module FastJsonapi
           polymorphic: polymorphic,
           conditional_proc: options[:if],
           transform_method: @transform_method,
+          meta: options[:meta],
           links: options[:links],
           lazy_load_data: options[:lazy_load_data]
         )

lib/fast_jsonapi/relationship.rb

@@ -1,6 +1,6 @@
 module FastJsonapi
   class Relationship
-    attr_reader :owner, :key, :name, :id_method_name, :record_type, :object_method_name, :object_block, :serializer, :relationship_type, :cached, :polymorphic, :conditional_proc, :transform_method, :links, :lazy_load_data
+    attr_reader :owner, :key, :name, :id_method_name, :record_type, :object_method_name, :object_block, :serializer, :relationship_type, :cached, :polymorphic, :conditional_proc, :transform_method, :links, :meta, :lazy_load_data
 
     def initialize(
       owner:,
@@ -17,6 +17,7 @@ module FastJsonapi
       conditional_proc:,
       transform_method:,
       links:,
+      meta:,
       lazy_load_data: false
     )
       @owner = owner
@@ -33,6 +34,7 @@ module FastJsonapi
       @conditional_proc = conditional_proc
       @transform_method = transform_method
       @links = links || {}
+      @meta = meta || {}
       @lazy_load_data = lazy_load_data
       @record_types_for = {}
       @serializers_for_name = {}
@@ -44,6 +46,8 @@ module FastJsonapi
 
         output_hash[key] = {}
         output_hash[key][:data] = ids_hash_from_record_and_relationship(record, serialization_params) || empty_case unless lazy_load_data && !included
+
+        add_meta_hash(record, serialization_params, output_hash) if meta.present?
         add_links_hash(record, serialization_params, output_hash) if links.present?
       end
     end
@@ -146,11 +150,19 @@ module FastJsonapi
                                    record.public_send(links)
                                  else
                                    links.each_with_object({}) do |(key, method), hash|
-                                     Link.new(key: key, method: method).serialize(record, params, hash)\
+                                     Link.new(key: key, method: method).serialize(record, params, hash)
                                    end
                                  end
     end
 
+    def add_meta_hash(record, params, output_hash)
+      output_hash[key][:meta] = if meta.is_a?(Proc)
+                                  FastJsonapi.call_proc(meta, record, params)
+                                else
+                                  meta
+                                end
+    end
+
     def run_key_transform(input)
       if transform_method.present?
         input.to_s.send(*transform_method).to_sym

spec/fixtures/movie.rb

@@ -66,6 +66,7 @@ class MovieSerializer
 
   has_many(
     :actors,
+    meta: proc { |record, _| { count: record.actors.length } },
     links: {
       actors_self: :url,
       related: ->(obj) { obj.url(obj) }

spec/integration/relationships_spec.rb

@@ -59,6 +59,13 @@ RSpec.describe FastJsonapi::ObjectSerializer do
         )
     end
 
+    describe 'has relationship meta' do
+      it do
+        expect(serialized['data']['relationships']['actors'])
+          .to have_meta('count' => movie.actors.length)
+      end
+    end
+
     context 'with include' do
       let(:params) do
         { include: [:actors] }