Added ActiveModel::Serializer#default_include class attribute to impl…

…ement rails-api#1333

Added test to cover new feature

lib/active_model/serializer/associations.rb

@@ -12,7 +12,7 @@ module Associations
 
       included do
         with_options instance_writer: false, instance_reader: true do |serializer|
-          serializer.class_attribute :_reflections
+          serializer.class_attribute :_reflections, :_default_include
           self._reflections ||= []
         end
 
@@ -65,6 +65,14 @@ def has_one(name, options = {}, &block) # rubocop:disable Style/PredicateName
           associate(HasOneReflection.new(name, options, block))
         end
 
+        # Set _default_include to the parsed value of +include+.
+        # @param includes value to be parsed by JSONAPI::IncludeDirective::Parser
+        # @return [void]
+        #
+        def default_include(include, options = {})
+          self._default_include = JSONAPI::IncludeDirective.new(include, options)
+        end
+
         private
 
         # Add reflection and define {name} accessor.
@@ -74,7 +82,7 @@ def has_one(name, options = {}, &block) # rubocop:disable Style/PredicateName
         # @api private
         #
         def associate(reflection)
-          self._reflections << reflection
+          _reflections << reflection
         end
       end
 
@@ -85,6 +93,8 @@ def associate(reflection)
       def associations(include_directive = ActiveModelSerializers.default_include_directive)
         return unless object
 
+        include_directive.merge!(_default_include) if _default_include
+
         Enumerator.new do |y|
           self.class._reflections.each do |reflection|
             next if reflection.excluded?(self)

test/serializers/default_include_test.rb

@@ -0,0 +1,52 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class DefaultIncludeTest < ActiveSupport::TestCase
+      class Blog < ActiveModelSerializers::Model
+        attr_accessor :id, :name, :posts
+      end
+      class Post < ActiveModelSerializers::Model
+        attr_accessor :id, :title, :author
+      end
+      class Author < ActiveModelSerializers::Model
+        attr_accessor :id, :name
+      end
+      class BlogSerializer < ActiveModel::Serializer
+        attributes :id
+        attribute :name, key: :title
+
+        has_many :posts
+      end
+      class PostSerializer < ActiveModel::Serializer
+        default_include [:author]
+        attributes :id, :title
+
+        has_one :author
+      end
+      class AuthorSerializer < ActiveModel::Serializer
+        attributes :id, :name
+      end
+
+      setup do
+        @authors = [Author.new(id: 1, name: 'Blog Author')]
+        @posts = [Post.new(id: 1, title: 'The first post', author: @authors.first), Post.new(id: 2, title: 'The second post', author: @authors.first)]
+        @blog = Blog.new(id: 2, name: 'The Blog', posts: @posts)
+        @serializer_instance = BlogSerializer.new(@blog)
+        @serializable = ActiveModelSerializers::SerializableResource.new(@blog, serializer: BlogSerializer, adapter: :attributes)
+        @expected_hash = {
+            id: 2,
+            title: 'The Blog',
+            posts: [
+                { id: 1, title: 'The first post', author: { id: 1, name: 'Blog Author'} },
+                { id: 2, title: 'The second post', author: { id: 1, name: 'Blog Author'} }
+            ] }
+      end
+
+      test '#default_include allows to include associations in the result of #serializable_hash' do
+        assert_equal @serializable.serializable_hash, @serializer_instance.serializable_hash
+        assert_equal @expected_hash, @serializer_instance.serializable_hash
+      end
+    end
+  end
+end