Ensure inheritance hooks run

I was seeing transient failures where adapters may not be registered.

e.g. https://travis-ci.org/rails-api/active_model_serializers/builds/77735382

Since we're using the Adapter, JsonApi, and Json classes
as namespaces, some of the conventions we use for modules don't apply.
Basically, we don't want to define the class anywhere besides itself.
Otherwise, the inherited hooks may not run, and some adapters may not
be registered.

For example:

If we have a class Api `class Api; end`
And Api is also used as a namespace for `Api::Product`
And the classes are defined in different files.

In one file:

```ruby
class Api
  autoload :Product
  def self.inherited(subclass)
    puts
    p [:inherited, subclass.name]
    puts
  end
end
```

And in another:

```ruby
class Api
  class Product < Api
    def sell_sell_sell!
      # TODO: sell
    end
  end
end
```

If we load the Api class file first, the inherited hook will be defined on the class
so that when we load the Api::Product class, we'll see the output:

```plain
[ :inherited, Api::Product]
```

However, if we load the Api::Product class first, since it defines the `Api` class
and then inherited from it, the Api file was never loaded, the hook never defined,
and thus never run.

By defining the class as `class Api::Product < Api` We ensure the the Api class
MUST be defined, and thus, the hook will be defined and run and so sunshine and unicorns.

Appendix:

The below would work, but triggers a circular reference warning.
It's also not recommended to mix require with autoload.

```ruby
require 'api'
class Api
  class Product < Api
    def sell_sell_sell!
      # TODO: sell
    end
  end
end
```

This failure scenario was introduced by removing the circular reference warnings in
https://github.com/rails-api/active_model_serializers/pull/1067

Style note:

To make diffs on the adapters smalleer and easier to read, I've maintained the same
identention that was in the original file.  I've decided to prefer ease of reading
the diff over style, esp. since we may later return to the preferred class declaration style.

 with '#' will be ignored, and an empty message aborts the commit.

.rubocop.yml

@@ -8,6 +8,39 @@ AllCops:
   DisplayCopNames: true
   DisplayStyleGuide: true
 
+Style/IndentationConsistency:
+  Exclude:
+    - lib/active_model/serializer/adapter/flatten_json.rb
+    - lib/active_model/serializer/adapter/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json.rb
+    - lib/active_model/serializer/adapter/json/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json_api.rb
+    - lib/active_model/serializer/adapter/json_api/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json_api/pagination_links.rb
+    - lib/active_model/serializer/adapter/null.rb
+
+Style/IndentationWidth:
+  Exclude:
+    - lib/active_model/serializer/adapter/flatten_json.rb
+    - lib/active_model/serializer/adapter/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json.rb
+    - lib/active_model/serializer/adapter/json/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json_api.rb
+    - lib/active_model/serializer/adapter/json_api/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json_api/pagination_links.rb
+    - lib/active_model/serializer/adapter/null.rb
+
+Style/AccessModifierIndentation:
+  Exclude:
+    - lib/active_model/serializer/adapter/flatten_json.rb
+    - lib/active_model/serializer/adapter/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json.rb
+    - lib/active_model/serializer/adapter/json/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json_api.rb
+    - lib/active_model/serializer/adapter/json_api/fragment_cache.rb
+    - lib/active_model/serializer/adapter/json_api/pagination_links.rb
+    - lib/active_model/serializer/adapter/null.rb
+
 Lint/NestedMethodDefinition:
   Enabled: false
   Exclude:
@@ -16,9 +49,6 @@ Lint/NestedMethodDefinition:
 Style/StringLiterals:
   EnforcedStyle: single_quotes
 
-Style/SpecialGlobalVars:
-  Enabled: false
-
 Metrics/AbcSize:
   Max: 35 # TODO: Lower to 15
 

lib/active_model/serializer/adapter.rb

@@ -5,11 +5,11 @@ module ActiveModel
       ADAPTER_MAP = {}
       private_constant :ADAPTER_MAP if defined?(private_constant)
       extend ActiveSupport::Autoload
-      require 'active_model/serializer/adapter/json'
-      require 'active_model/serializer/adapter/json_api'
-      autoload :FlattenJson
-      autoload :Null
       autoload :FragmentCache
+      autoload :Json
+      autoload :JsonApi
+      autoload :Null
+      autoload :FlattenJson
 
       def self.create(resource, options = {})
         override = options.delete(:adapter)
@@ -60,14 +60,14 @@ module ActiveModel
             register(adapter_name, adapter_class)
             adapter_class
           }
-        rescue ArgumentError
+        rescue ArgumentError => e
           failure_message =
             "Unknown adapter: #{adapter.inspect}. Valid adapters are: #{adapters}"
-          raise UnknownAdapterError, failure_message, $!.backtrace
-        rescue NameError
+          raise UnknownAdapterError, failure_message, e.backtrace
+        rescue NameError => e
           failure_message =
-            "NameError: #{$!.message}. Unknown adapter: #{adapter.inspect}. Valid adapters are: #{adapters}"
-          raise UnknownAdapterError, failure_message, $!.backtrace
+            "NameError: #{e.message}. Unknown adapter: #{adapter.inspect}. Valid adapters are: #{adapters}"
+          raise UnknownAdapterError, failure_message, e.backtrace
         end
 
         # @api private

lib/active_model/serializer/adapter/flatten_json.rb

@@ -1,7 +1,4 @@
-module ActiveModel
-  class Serializer
-    class Adapter
-      class FlattenJson < Json
+class ActiveModel::Serializer::Adapter::FlattenJson < ActiveModel::Serializer::Adapter::Json
         def serializable_hash(options = {})
           super
           @result
@@ -14,6 +11,3 @@ module ActiveModel
           json
         end
 end
-    end
-  end
-end

lib/active_model/serializer/adapter/fragment_cache.rb

@@ -1,7 +1,4 @@
-module ActiveModel
-  class Serializer
-    class Adapter
-      class FragmentCache
+class ActiveModel::Serializer::Adapter::FragmentCache
         attr_reader :serializer
 
         def initialize(adapter, serializer, options)
@@ -76,6 +73,3 @@ module ActiveModel
           name.gsub('::', '_')
         end
 end
-    end
-  end
-end

lib/active_model/serializer/adapter/json.rb

@@ -1,9 +1,7 @@
-require 'active_model/serializer/adapter/json/fragment_cache'
+class ActiveModel::Serializer::Adapter::Json < ActiveModel::Serializer::Adapter
+        extend ActiveSupport::Autoload
+        autoload :FragmentCache
 
-module ActiveModel
-  class Serializer
-    class Adapter
-      class Json < Adapter
         def serializable_hash(options = nil)
           options ||= {}
           if serializer.respond_to?(:each)
@@ -44,9 +42,6 @@ module ActiveModel
         end
 
         def fragment_cache(cached_hash, non_cached_hash)
-          Json::FragmentCache.new().fragment_cache(cached_hash, non_cached_hash)
-        end
-      end
-    end
+          ActiveModel::Serializer::Adapter::Json::FragmentCache.new().fragment_cache(cached_hash, non_cached_hash)
         end
 end

lib/active_model/serializer/adapter/json/fragment_cache.rb

@@ -1,14 +1,5 @@
-require 'active_model/serializer/adapter/fragment_cache'
-module ActiveModel
-  class Serializer
-    class Adapter
-      class Json < Adapter
-        class FragmentCache
+class ActiveModel::Serializer::Adapter::Json::FragmentCache
           def fragment_cache(cached_hash, non_cached_hash)
             non_cached_hash.merge cached_hash
           end
 end
-      end
-    end
-  end
-end

lib/active_model/serializer/adapter/json_api.rb

@@ -1,10 +1,8 @@
-require 'active_model/serializer/adapter/json_api/fragment_cache'
-require 'active_model/serializer/adapter/json_api/pagination_links'
+class ActiveModel::Serializer::Adapter::JsonApi < ActiveModel::Serializer::Adapter
+        extend ActiveSupport::Autoload
+        autoload :PaginationLinks
+        autoload :FragmentCache
 
-module ActiveModel
-  class Serializer
-    class Adapter
-      class JsonApi < Adapter
         def initialize(serializer, options = {})
           super
           @hash = { data: [] }
@@ -49,7 +47,7 @@ module ActiveModel
 
         def fragment_cache(cached_hash, non_cached_hash)
           root = false if @options.include?(:include)
-          JsonApi::FragmentCache.new.fragment_cache(root, cached_hash, non_cached_hash)
+          ActiveModel::Serializer::Adapter::JsonApi::FragmentCache.new().fragment_cache(root, cached_hash, non_cached_hash)
         end
 
         private
@@ -62,6 +60,12 @@ module ActiveModel
           end
         end
 
+        def add_relationships(resource, name, serializers)
+          resource[:relationships] ||= {}
+          resource[:relationships][name] ||= { data: [] }
+          resource[:relationships][name][:data] += serializers.map { |serializer| { type: serializer.json_api_type, id: serializer.id.to_s } }
+        end
+
         def resource_identifier_id_for(serializer)
           if serializer.respond_to?(:id)
             serializer.id
@@ -161,8 +165,8 @@ module ActiveModel
           @hash[:links] = add_pagination_links(links, collection, options) if paginated?(collection)
         end
 
-        def add_pagination_links(links, collection, options)
-          pagination_links = JsonApi::PaginationLinks.new(collection, options[:context]).serializable_hash(options)
+        def add_pagination_links(links, resources, options)
+          pagination_links = ActiveModel::Serializer::Adapter::JsonApi::PaginationLinks.new(resources, options[:context]).serializable_hash(options)
           links.update(pagination_links)
         end
 
@@ -172,6 +176,3 @@ module ActiveModel
             collection.respond_to?(:size)
         end
 end
-    end
-  end
-end

lib/active_model/serializer/adapter/json_api/fragment_cache.rb

@@ -1,9 +1,4 @@
-require 'active_model/serializer/adapter/fragment_cache'
-module ActiveModel
-  class Serializer
-    class Adapter
-      class JsonApi < Adapter
-        class FragmentCache
+class ActiveModel::Serializer::Adapter::JsonApi::FragmentCache
           def fragment_cache(root, cached_hash, non_cached_hash)
             hash              = {}
             core_cached       = cached_hash.first
@@ -16,7 +11,3 @@ module ActiveModel
             hash.deep_merge no_root_non_cache.deep_merge no_root_cache
           end
 end
-      end
-    end
-  end
-end

lib/active_model/serializer/adapter/json_api/pagination_links.rb

@@ -1,8 +1,4 @@
-module ActiveModel
-  class Serializer
-    class Adapter
-      class JsonApi < Adapter
-        class PaginationLinks
+class ActiveModel::Serializer::Adapter::JsonApi::PaginationLinks
           FIRST_PAGE = 1
 
           attr_reader :collection, :context
@@ -52,7 +48,3 @@ module ActiveModel
             @query_parameters ||= context.query_parameters
           end
 end
-      end
-    end
-  end
-end

lib/active_model/serializer/adapter/null.rb

@@ -1,11 +1,5 @@
-module ActiveModel
-  class Serializer
-    class Adapter
-      class Null < Adapter
+class ActiveModel::Serializer::Adapter::Null < ActiveModel::Serializer::Adapter
         def serializable_hash(options = nil)
           {}
         end
 end
-    end
-  end
-end

test/serializers/adapter_for_test.rb

@@ -5,6 +5,11 @@ module ActiveModel
 
       def setup
         @previous_adapter = ActiveModel::Serializer.config.adapter
+        # Eager load adapters
+        ActiveModel::Serializer::Adapter.eager_load!
+        [:json_api, :flatten_json, :null, :json].each do |adapter_name|
+          ActiveModel::Serializer::Adapter.lookup(adapter_name)
+        end
       end
 
       def teardown

test/test_helper.rb

@@ -39,15 +39,6 @@ else
 end
 
 require 'active_model_serializers'
-# eager load autoloaded adapters
-# rubocop:disable Lint/Void
-require 'active_model/serializer/adapter'
-ActiveModel::Serializer::Adapter::Null
-ActiveModel::Serializer::Adapter::Json
-ActiveModel::Serializer::Adapter::FlattenJson
-ActiveModel::Serializer::Adapter::JsonApi
-# rubocop:enable Lint/Void
-require 'active_model/serializer/adapter'
 
 require 'support/stream_capture'