ActiveModel support for the :include serialization option

This commit moves support for the :include serialization option for
serializing associated objects out of ActiveRecord in into ActiveModel.
The following methods support the :include option:

  * serializable_hash
  * to_json
  * to_xml

Instances must respond to methods named by the values of the :includes
array (or keys of the :includes hash). If an association method returns
an object that is_a?(Enumerable) (which AR has_many associations do), it
is assumed to be a collection association, and its elements must respond
to :serializable_hash. Otherwise it must respond to :serializable_hash
itself.

While here, fix #858, XmlSerializer should not singularize already
singular association names.

2 files changed, 72 insertions, 1 deletions

diff --git a/activemodel/lib/active_model/serialization.rb b/activemodel/lib/active_model/serialization.rb
index 0b4067257e..9260c5082d 100644
--- a/activemodel/lib/active_model/serialization.rb
+++ b/activemodel/lib/active_model/serialization.rb
@@ -77,7 +77,38 @@ module ActiveModel
       end
 
       method_names = Array.wrap(options[:methods]).select { |n| respond_to?(n) }
-      Hash[(attribute_names + method_names).map { |n| [n, send(n)] }]
+      hash = Hash[(attribute_names + method_names).map { |n| [n, send(n)] }]
+
+      serializable_add_includes(options) do |association, records, opts|
+        hash[association] = if records.is_a?(Enumerable)
+          records.map { |a| a.serializable_hash(opts) }
+        else
+          records.serializable_hash(opts)
+        end
+      end
+
+      hash
     end
+
+    private
+      # Add associations specified via the <tt>:include</tt> option.
+      #
+      # Expects a block that takes as arguments:
+      #   +association+ - name of the association
+      #   +records+     - the association record(s) to be serialized
+      #   +opts+        - options for the association records
+      def serializable_add_includes(options = {})
+        return unless include = options[:include]
+
+        unless include.is_a?(Hash)
+          include = Hash[Array.wrap(include).map { |n| [n, {}] }]
+        end
+
+        include.each do |association, opts|
+          if records = send(association)
+            yield association, records, opts
+          end
+        end
+      end
   end
 end
diff --git a/activemodel/lib/active_model/serializers/xml.rb b/activemodel/lib/active_model/serializers/xml.rb
index 9812af43d6..64dda3bcee 100644
--- a/activemodel/lib/active_model/serializers/xml.rb
+++ b/activemodel/lib/active_model/serializers/xml.rb
@@ -101,6 +101,7 @@ module ActiveModel
 
           @builder.tag!(*args) do
             add_attributes_and_methods
+            add_includes
             add_extra_behavior
             add_procs
             yield @builder if block_given?
@@ -120,6 +121,45 @@ module ActiveModel
           end
         end
 
+        def add_includes
+          @serializable.send(:serializable_add_includes, options) do |association, records, opts|
+            add_associations(association, records, opts)
+          end
+        end
+
+        # TODO This can likely be cleaned up to simple use ActiveSupport::XmlMini.to_tag as well.
+        def add_associations(association, records, opts)
+          merged_options = opts.merge(options.slice(:builder, :indent))
+          merged_options[:skip_instruct] = true
+
+          if records.is_a?(Enumerable)
+            tag  = ActiveSupport::XmlMini.rename_key(association.to_s, options)
+            type = options[:skip_types] ? { } : {:type => "array"}
+            association_name = association.to_s.singularize
+            merged_options[:root] = association_name
+
+            if records.empty?
+              @builder.tag!(tag, type)
+            else
+              @builder.tag!(tag, type) do
+                records.each do |record|
+                  if options[:skip_types]
+                    record_type = {}
+                  else
+                    record_class = (record.class.to_s.underscore == association_name) ? nil : record.class.name
+                    record_type = {:type => record_class}
+                  end
+
+                  record.to_xml merged_options.merge(record_type)
+                end
+              end
+            end
+          else
+            merged_options[:root] = association.to_s
+            records.to_xml(merged_options)
+          end
+        end
+
         def add_procs
           if procs = options.delete(:procs)
             Array.wrap(procs).each do |proc|