When objects are sideloaded multiple times, serialize them only once

To achieve this, we make the following change when sideloading: Instead
of serializing associations and discarding duplicate *hashes*, we
memorize the *objects* (records) that we have already serialized, and
only serialize those that are new.

This change is mostly transparent, and brings down serialization time
from 3.1 seconds to 1.0 seconds on my set of sample data.

There is one change in the behavior: If you sideload the same object
multiple times, and it yields different hashes, like so:

    embed :ids, include: true
    has_many :comments
    has_many :recent_comments, root: comments, serializer: CommentShortSerializer

then previously, it would be included multiple times, whereas now, the
first hash wins. (I haven't actually tested this.) I don't know that
either option is preferable. It's not covered by the test suite, and I
think it's an edge case that is OK to ignore entirely.

lib/active_model/serializer.rb

@@ -310,7 +310,7 @@ module ActiveModel
         if association.embed_in_root? && hash.nil?
           raise IncludeError.new(self.class, association.name)
         elsif association.embed_in_root? && association.embeddable?
-          merge_association hash, association.root, association.serialize_many, unique_values
+          merge_association hash, association.root, association.serializables, unique_values
         end
       elsif association.embed_objects?
         node[association.key] = association.serialize
@@ -326,13 +326,15 @@ module ActiveModel
     # a unique list of all of the objects that are already in the Array. This
     # avoids the need to scan through the Array looking for entries every time
     # we want to merge a new list of values.
-    def merge_association(hash, key, value, unique_values)
-      if current_value = unique_values[key]
-        current_value.merge! value
-        hash[key] = current_value.to_a
-      elsif value
-        hash[key] = value
-        unique_values[key] = OrderedSet.new(value)
+    def merge_association(hash, key, serializables, unique_values)
+      already_serialized = (unique_values[key] ||= {})
+      serializable_hashes = (hash[key] ||= [])
+
+      serializables.each do |serializable|
+        unless already_serialized.include? serializable.object
+          already_serialized[serializable.object] = true
+          serializable_hashes << serializable.serializable_hash
+        end
       end
     end
 

lib/active_model/serializer/associations.rb

@@ -99,7 +99,12 @@ module ActiveModel
             find_serializable(item).serializable_hash
           end
         end
-        alias serialize_many serialize
+
+        def serializables
+          associated_object.map do |item|
+            find_serializable(item)
+          end
+        end
 
         def serialize_ids
           # Use pluck or select_columns if available
@@ -149,9 +154,9 @@ module ActiveModel
           end
         end
 
-        def serialize_many
+        def serializables
           object = associated_object
-          value = object && find_serializable(object).serializable_hash
+          value = object && find_serializable(object)
           value ? [value] : []
         end
 

test/association_test.rb

@@ -284,6 +284,29 @@ class AssociationTest < ActiveModel::TestCase
         ]
       }, @root_hash)
     end
+
+    def test_embed_ids_include_true_does_not_serialize_multiple_times
+      @post.recent_comment = @comment
+
+      @post_serializer_class.class_eval do
+        has_one :comment, :embed => :ids, :include => true
+        has_one :recent_comment, :embed => :ids, :include => true, :root => :comments
+      end
+
+      # Count how often the @comment record is serialized.
+      serialized_times = 0
+      @comment.class_eval do
+        define_method :read_attribute_for_serialization, lambda { |name|
+          serialized_times += 1 if name == :body
+          super(name)
+        }
+      end
+
+      include_bare! :comment
+      include_bare! :recent_comment
+
+      assert_equal 1, serialized_times
+    end
   end
 
   class InclusionTest < AssociationTest

test/serializer_test.rb

@@ -73,11 +73,13 @@ class SerializerTest < ActiveModel::TestCase
 
   class CommentSerializer
     def initialize(comment, options={})
-      @comment = comment
+      @object = comment
     end
 
+    attr_reader :object
+
     def serializable_hash
-      { :title => @comment.read_attribute_for_serialization(:title) }
+      { :title => @object.read_attribute_for_serialization(:title) }
     end
 
     def as_json(options=nil)