Provide key case translation

lib/action_controller/serialization.rb

@@ -56,7 +56,9 @@ module ActionController
 
     [:_render_option_json, :_render_with_renderer_json].each do |renderer_method|
       define_method renderer_method do |resource, options|
-        options.fetch(:serialization_context) { options[:serialization_context] = ActiveModelSerializers::SerializationContext.new(request) }
+        options.fetch(:serialization_context) do
+          options[:serialization_context] = ActiveModelSerializers::SerializationContext.new(request, options)
+        end
         serializable_resource = get_serializer(resource, options)
         super(serializable_resource, options)
       end

lib/active_model/serializer/configuration.rb

@@ -26,6 +26,7 @@ module ActiveModel
         # Make JSON API top-level jsonapi member opt-in
         # ref: http://jsonapi.org/format/#document-top-level
         config.jsonapi_include_toplevel_object = false
+        config.key_transform = nil
 
         config.schema_path = 'test/support/schemas'
       end

lib/active_model_serializers/adapter/base.rb

@@ -1,3 +1,5 @@
+require 'active_model_serializers/key_transform'
+
 module ActiveModelSerializers
   module Adapter
     class Base
@@ -51,6 +53,27 @@ module ActiveModelSerializers
         json[meta_key] = meta unless meta.blank?
         json
       end
+
+      def default_key_transform
+        :unaltered
+      end
+
+      # Determines the transform to use in order of precedence:
+      #   serialization context, global config, adapter default.
+      #
+      # @param serialization_context [Object] the SerializationContext
+      # @return [Symbol] the transform to use
+      def key_transform(serialization_context)
+        serialization_context.key_transform ||
+        ActiveModelSerializers.config.key_transform ||
+        default_key_transform
+      end
+
+      def transform_key_casing!(value, serialization_context)
+        return value unless serialization_context
+        transform = key_transform(serialization_context)
+        KeyTransform.send(transform, value)
+      end
     end
   end
 end

lib/active_model_serializers/adapter/json.rb

@@ -3,7 +3,8 @@ module ActiveModelSerializers
     class Json < Base
       def serializable_hash(options = nil)
         options ||= {}
-        { root => Attributes.new(serializer, instance_options).serializable_hash(options) }
+        serialized_hash = { root => Attributes.new(serializer, instance_options).serializable_hash(options) }
+        transform_key_casing!(serialized_hash, options[:serialization_context])
       end
     end
   end

lib/active_model_serializers/adapter/json_api.rb

@@ -37,15 +37,20 @@ module ActiveModelSerializers
         @fieldset = options[:fieldset] || ActiveModel::Serializer::Fieldset.new(options.delete(:fields))
       end
 
+      def default_key_transform
+        :dashed
+      end
+
       # {http://jsonapi.org/format/#crud Requests are transactional, i.e. success or failure}
       # {http://jsonapi.org/format/#document-top-level data and errors MUST NOT coexist in the same document.}
       def serializable_hash(options = nil)
         options ||= {}
-        if serializer.success?
-          success_document(options)
-        else
-          failure_document
-        end
+        document = if serializer.success?
+                     success_document(options)
+                   else
+                     failure_document
+                   end
+        transform_key_casing!(document, options[:serialization_context])
       end
 
       # {http://jsonapi.org/format/#document-top-level Primary data}

lib/active_model_serializers/key_transform.rb

@@ -0,0 +1,40 @@
+require 'active_support/core_ext/hash/keys'
+
+module ActiveModelSerializers
+  module KeyTransform
+    module_function
+
+    # Transforms keys to UpperCamelCase or PascalCase.
+    #
+    # @example:
+    #    "some_key" => "SomeKey",
+    # @see {https://github.com/rails/rails/blob/master/activesupport/lib/active_support/inflector/methods.rb#L66-L76 ActiveSupport::Inflector.camelize}
+    def camel(hash)
+      hash.deep_transform_keys! { |key| key.to_s.camelize.to_sym }
+    end
+
+    # Transforms keys to camelCase.
+    #
+    # @example:
+    #    "some_key" => "someKey",
+    # @see {https://github.com/rails/rails/blob/master/activesupport/lib/active_support/inflector/methods.rb#L66-L76 ActiveSupport::Inflector.camelize}
+    def camel_lower(hash)
+      hash.deep_transform_keys! { |key| key.to_s.camelize(:lower).to_sym }
+    end
+
+    # Transforms keys to dashed-case.
+    # This is the default case for the JsonApi adapter.
+    #
+    # @example:
+    #    "some_key" => "some-key",
+    # @see {https://github.com/rails/rails/blob/master/activesupport/lib/active_support/inflector/methods.rb#L185-L187 ActiveSupport::Inflector.dasherize}
+    def dashed(hash)
+      hash.deep_transform_keys! { |key| key.to_s.dasherize.to_sym }
+    end
+
+    # Returns the hash unaltered
+    def unaltered(hash)
+      hash
+    end
+  end
+end

lib/active_model_serializers/serialization_context.rb

@@ -4,13 +4,14 @@ module ActiveModelSerializers
       attr_writer :url_helpers, :default_url_options
     end
 
-    attr_reader :request_url, :query_parameters
+    attr_reader :request_url, :query_parameters, :key_transform
 
     def initialize(request, options = {})
       @request_url = request.original_url[/\A[^?]+/]
       @query_parameters = request.query_parameters
       @url_helpers = options.delete(:url_helpers) || self.class.url_helpers
       @default_url_options = options.delete(:default_url_options) || self.class.default_url_options
+      @key_transform = options.delete(:key_transform)
     end
 
     def self.url_helpers