Explicitly set serializer for associations

README.md

@@ -189,6 +189,12 @@ The `has_many` and `belongs_to` declarations describe relationships between
 resources. By default, when you serialize a `Post`, you will get its `Comment`s
 as well.
 
+You may also use the `:serializer` option to specify a custom serializer class, for example:
+
+```ruby
+  has_many :comments, serializer: CommentPreviewSerializer
+```
+
 The `url` declaration describes which named routes to use while generating URLs
 for your JSON. Not every adapter will require URLs.
 

lib/active_model/serializer.rb

@@ -23,7 +23,7 @@ def self.attributes(*attrs)
 
       attrs.each do |attr|
         define_method attr do
-          object.read_attribute_for_serialization(attr)
+          object && object.read_attribute_for_serialization(attr)
         end unless method_defined?(attr)
       end
     end
@@ -67,7 +67,7 @@ def self.associate(type, attrs) #:nodoc:
           end
         end
 
-        self._associations[attr] = {type: type, options: options}
+        self._associations[attr] = {type: type, association_options: options}
       end
     end
 
@@ -79,11 +79,13 @@ def self.urls(*attrs)
       @_urls.concat attrs
     end
 
-    def self.serializer_for(resource)
+    def self.serializer_for(resource, options = {})
       if resource.respond_to?(:to_ary)
         config.array_serializer
       else
-        get_serializer_for(resource.class)
+        options
+          .fetch(:association_options, {})
+          .fetch(:serializer, get_serializer_for(resource.class))
       end
     end
 
@@ -138,16 +140,27 @@ def attributes(options = {})
 
     def each_association(&block)
       self.class._associations.dup.each do |name, options|
+        next unless object
         association = object.send(name)
-        serializer_class = ActiveModel::Serializer.serializer_for(association)
-        serializer = serializer_class.new(association) if serializer_class
+        serializer_class = ActiveModel::Serializer.serializer_for(association, options)
+        serializer = serializer_class.new(
+          association,
+          serializer_from_options(options)
+        ) if serializer_class
 
         if block_given?
-          block.call(name, serializer, options[:options])
+          block.call(name, serializer, options[:association_options])
         end
       end
     end
 
+    def serializer_from_options(options)
+      opts = {}
+      serializer = options.fetch(:options, {}).fetch(:serializer, nil)
+      opts[:serializer] = serializer if serializer
+      opts
+    end
+
     private
 
     def self.get_serializer_for(klass)

lib/active_model/serializer/adapter/json_api.rb

@@ -18,7 +18,6 @@ def serializable_hash(options = {})
             end
           else
             @hash[@root] = attributes_for_serializer(serializer, @options)
-
             add_resource_links(@hash[@root], serializer)
           end
 
@@ -46,7 +45,7 @@ def add_link(resource, name, serializer)
           resource[:links] ||= {}
           resource[:links][name] = nil
 
-          if serializer
+          if serializer && serializer.object
             type = serialized_object_type(serializer)
             if name.to_s == type || !type
               resource[:links][name] = serializer.id.to_s

test/adapter/json_api/has_many_explicit_serializer_test.rb

@@ -0,0 +1,65 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApi
+        # Test 'has_many :assocs, serializer: AssocXSerializer'
+        class HasManyExplicitSerializerTest < Minitest::Test
+          def setup
+            @post = Post.new(title: 'New Post', body: 'Body')
+            @author = Author.new(name: 'Jane Blogger')
+            @author.posts = [@post]
+            @post.author = @author
+            @first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
+            @second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
+            @post.comments = [@first_comment, @second_comment]
+            @first_comment.post = @post
+            @first_comment.author = nil
+            @second_comment.post = @post
+            @second_comment.author = nil
+
+            @serializer = PostPreviewSerializer.new(@post)
+            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(
+              @serializer,
+              include: 'comments,author'
+            )
+          end
+
+          def test_includes_comment_ids
+            assert_equal(['1', '2'],
+                         @adapter.serializable_hash[:posts][:links][:comments])
+          end
+
+          def test_includes_linked_comments
+            assert_equal([{ id: '1', body: "ZOMG A COMMENT", links: { post: @post.id.to_s, author: nil }},
+                          { id: '2', body: "ZOMG ANOTHER COMMENT", links: { post: @post.id.to_s, author: nil }}],
+                         @adapter.serializable_hash[:linked][:comments])
+          end
+
+          def test_includes_author_id
+            assert_equal(@author.id.to_s,
+                         @adapter.serializable_hash[:posts][:links][:author])
+          end
+
+          def test_includes_linked_authors
+            assert_equal([{ id: @author.id.to_s, links: { posts: [@post.id.to_s] } }],
+                         @adapter.serializable_hash[:linked][:authors])
+          end
+
+          def test_explicit_serializer_with_null_resource
+            @post.author = nil
+            assert_equal(nil,
+                         @adapter.serializable_hash[:posts][:links][:author])
+          end
+
+          def test_explicit_serializer_with_null_collection
+            @post.comments = []
+            assert_equal([],
+                         @adapter.serializable_hash[:posts][:links][:comments])
+          end
+        end
+      end
+    end
+  end
+end

test/fixtures/poro.rb

@@ -100,3 +100,26 @@ def json_key
   attribute :id
   attribute :name, key: :title
 end
+
+CommentPreviewSerializer = Class.new(ActiveModel::Serializer) do
+  attributes :id
+
+  belongs_to :post
+end
+
+AuthorPreviewSerializer = Class.new(ActiveModel::Serializer) do
+  attributes :id
+
+  has_many :posts
+end
+
+PostPreviewSerializer = Class.new(ActiveModel::Serializer) do
+  def self.root_name
+    'posts'
+  end
+
+  attributes :title, :body, :id
+
+  has_many :comments, serializer: CommentPreviewSerializer
+  belongs_to :author, serializer: AuthorPreviewSerializer
+end

test/serializers/associations_test.rb

@@ -42,9 +42,9 @@ def setup
 
       def test_has_many
         assert_equal(
-          { posts: { type: :has_many, options: { embed: :ids } },
-            roles: { type: :has_many, options: { embed: :ids } },
-            bio: { type: :belongs_to, options: {} } },
+          { posts: { type: :has_many, association_options: { embed: :ids } },
+            roles: { type: :has_many, association_options: { embed: :ids } },
+            bio: { type: :belongs_to, association_options: {} } },
           @author_serializer.class._associations
         )
         @author_serializer.each_association do |name, serializer, options|
@@ -64,7 +64,7 @@ def test_has_many
       end
 
       def test_has_one
-        assert_equal({post: {type: :belongs_to, options: {}}, :author=>{:type=>:belongs_to, :options=>{}}}, @comment_serializer.class._associations)
+        assert_equal({post: {type: :belongs_to, association_options: {}}, :author=>{:type=>:belongs_to, :association_options=>{}}}, @comment_serializer.class._associations)
         @comment_serializer.each_association do |name, serializer, options|
           if name == :post
             assert_equal({}, options)