WIP: Add ability to define polymorphic serializers

This is done by adding a hash of key/value pairs matching a class type
to its serializer. It allows the ability to use something other than the
default serializer for polymorphic relationships.

README.md

@@ -805,6 +805,29 @@ end
 }
 ```
 
+You can also define serializers for a polymorphic relationship like so:
+
+
+```ruby
+class PostSerializer < ActiveModel::Serializer
+  attributes :id, :title
+  has_many :attachments, polymorphic: true, each_serializer: { video: SpecialVideoSerializer, audio: AudioSummarySerializer }
+end
+
+```
+
+Or, for `has_one` relationships:
+
+```ruby
+class PostSerializer < ActiveModel::Serializer
+  attributes :id, :title
+  has_one :attachment, polymorphic: true, serializer: { video: SpecialVideoSerializer, audio: AudioSummarySerializer }
+end
+```
+
+In this case, if the `attachment` is a Video object, instead of using the
+standard VideoSerializer, the SpecialVideoSerializer will be used. Any other
+objects not defined by the hash will use the default serializers.
 
 ## Customizing Scope
 

lib/active_model/array_serializer.rb

@@ -18,12 +18,13 @@ def initialize(object, options={})
       @polymorphic   = options.fetch(:polymorphic, false)
       @meta_key        = options[:meta_key] || :meta
       @meta            = options[@meta_key]
-      @each_serializer = options[:each_serializer]
       @resource_name   = options[:resource_name]
       @only            = options[:only] ? Array(options[:only]) : nil
       @except          = options[:except] ? Array(options[:except]) : nil
       @namespace       = options[:namespace]
       @key_format      = options[:key_format] || options[:each_serializer].try(:key_format)
+      @each_serializer = options[:each_serializer]
+      @each_serializer_from_options = @each_serializer.is_a?(Hash) ? nil : @each_serializer
     end
     attr_accessor :object, :scope, :root, :meta_key, :meta, :key_format
 
@@ -34,7 +35,7 @@ def json_key
     end
 
     def serializer_for(item)
-      serializer_class = @each_serializer || Serializer.serializer_for(item, namespace: @namespace) || DefaultSerializer
+      serializer_class = @each_serializer_from_options || Serializer.serializer_for(item, namespace: @namespace) || DefaultSerializer
       serializer_class.new(item, scope: scope, key_format: key_format, only: @only, except: @except, polymorphic: @polymorphic, namespace: @namespace)
     end
 

lib/active_model/polymorphic_array_serializer.rb

@@ -0,0 +1,13 @@
+module ActiveModel
+  class PolymorphicArraySerializer < ArraySerializer
+    def serializer_for(item)
+      if @each_serializer.is_a?(Hash) && polymorphic?
+        polymorphic_object_key = item.class.name.underscore.to_sym
+        return nil unless @each_serializer.has_key?(polymorphic_object_key)
+        @each_serializer[polymorphic_object_key]
+      else
+        super
+      end
+    end
+  end
+end

lib/active_model/serializer/association.rb

@@ -24,8 +24,14 @@ def initialize(name, options={})
         @embed_in_root_key = options.fetch(:embed_in_root_key) { CONFIG.embed_in_root_key }
         @embed_namespace = options.fetch(:embed_namespace) { CONFIG.embed_namespace }
 
-        serializer = @options[:serializer]
-        @serializer_from_options = serializer.is_a?(String) ? serializer.constantize : serializer
+        @serializer = options[:serializer]
+        @serializer_from_options = if @serializer.is_a?(Hash)
+          nil
+        elsif @serializer.is_a?(String)
+          @serializer.constantize
+        else
+          @serializer
+        end
       end
 
       attr_reader :name, :embed_ids, :embed_objects, :polymorphic
@@ -43,7 +49,13 @@ def embed=(embed)
       end
 
       def serializer_from_object(object, options = {})
-        Serializer.serializer_for(object, options)
+        if @serializer.is_a?(Hash) && polymorphic?
+          polymorphic_object_key = object.class.name.underscore.to_sym
+          return nil unless @serializer.has_key?(polymorphic_object_key)
+          @serializer[polymorphic_object_key]
+        else
+          Serializer.serializer_for(object, options)
+        end
       end
 
       def default_serializer

lib/active_model/serializer/association/has_many.rb

@@ -1,3 +1,5 @@
+require 'active_model/polymorphic_array_serializer'
+
 module ActiveModel
   class Serializer
     class Association
@@ -13,7 +15,7 @@ def initialize(name, *args)
 
         def serializer_class(object, _)
           if use_array_serializer?
-            ArraySerializer
+            polymorphic? ? PolymorphicArraySerializer : ArraySerializer
           else
             serializer_from_options
           end

lib/active_model/serializer/association/has_one.rb

@@ -22,4 +22,4 @@ def build_serializer(object, options = {})
       end
     end
   end
-end
+end