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
rails-api#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.

Author: bf4

.rubocop.yml

@@ -8,6 +8,42 @@ AllCops:
   DisplayCopNames: true
   DisplayStyleGuide: true
 
+Style/IndentationConsistency:
+  Exclude:
+    - lib/active_model/serializer/adapter.rb
+    - 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.rb
+    - 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.rb
+    - 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:

lib/active_model/serializable_resource.rb

@@ -76,7 +76,7 @@ def serializer?
     private
 
     ActiveModelSerializers.silence_warnings do
-    attr_reader :resource, :adapter_opts, :serializer_opts
+      attr_reader :resource, :adapter_opts, :serializer_opts
     end
   end
 end

lib/active_model/serializer/adapter.rb

@@ -5,11 +5,11 @@ class Adapter
       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)

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
@@ -13,7 +10,4 @@ def serializable_hash(options = {})
         def include_meta(json)
           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)
@@ -75,7 +72,4 @@ def fragment_serializer(name, klass)
         def to_valid_const_name(name)
           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 @@ def serializable_hash(options = nil)
         end
 
         def fragment_cache(cached_hash, non_cached_hash)
-          Json::FragmentCache.new().fragment_cache(cached_hash, non_cached_hash)
+          ActiveModel::Serializer::Adapter::Json::FragmentCache.new().fragment_cache(cached_hash, non_cached_hash)
         end
-      end
-    end
-  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: [] }
@@ -39,11 +37,9 @@ def serializable_hash(options = nil)
 
         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
-
         def add_relationships(resource, name, serializers)
           resource[:relationships] ||= {}
           resource[:relationships][name] ||= { data: [] }
@@ -169,17 +165,14 @@ def add_links(options)
           end
         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
 
-        def is_paginated?(collection)
-          collection.respond_to?(:current_page) &&
-            collection.respond_to?(:total_pages) &&
-            collection.respond_to?(:size)
+        def is_paginated?(resource)
+          resource.respond_to?(:current_page) &&
+            resource.respond_to?(:total_pages) &&
+            resource.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
@@ -15,8 +10,4 @@ def fragment_cache(root, cached_hash, non_cached_hash)
 
             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
@@ -51,8 +47,4 @@ def original_url
           def query_parameters
             @query_parameters ||= context.query_parameters
           end
-        end
-      end
-    end
-  end
 end