Adding 'embed_key' option to allow embedding attributes other than ID

README.md

@@ -535,6 +535,34 @@ This would generate JSON that would look like this:
 }
 ```
 
+You can also specify a different attribute to use rather than the ID of the
+objects:
+
+```ruby
+class PostSerializer < ActiveModel::Serializer
+  embed :ids, :include => true
+
+  attributes :id, :title, :body
+  has_many :comments, :embed_key => :external_id
+end
+```
+
+This would generate JSON that would look like this:
+
+```json
+{
+  "post": {
+    "id": 1,
+    "title": "New post",
+    "body": "A body!",
+    "comment_ids": [ "COMM001" ]
+  },
+  "comments": [
+    { "id": 1, "external_id": "COMM001", "body": "what a dumb post" }
+  ]
+}
+```
+
 **NOTE**: The `embed :ids` mechanism is primary useful for clients that process
 data in bulk and load it into a local store. For these clients, the ability to
 easily see all of the data per type, rather than having to recursively scan the

lib/active_model/serializer/associations.rb

@@ -103,6 +103,14 @@ module ActiveModel
           end
         end
 
+        def embed_key
+          if key = option(:embed_key)
+            key
+          else
+            :id
+          end
+        end
+
         def serialize
           associated_object.map do |item|
             find_serializable(item).serializable_hash
@@ -120,11 +128,11 @@ module ActiveModel
           # return collection.ids if collection.respond_to?(:ids)
           ids_key = "#{key.to_s.singularize}_ids"
 
-          if !option(:include) && source_serializer.object.respond_to?(ids_key)
+          if !option(:include) && !option(:embed_key) && source_serializer.object.respond_to?(ids_key)
             source_serializer.object.send(ids_key)
           else
             associated_object.map do |item|
-              item.read_attribute_for_serialization(:id)
+              item.read_attribute_for_serialization(embed_key)
             end
           end
         end
@@ -163,6 +171,14 @@ module ActiveModel
           end
         end
 
+        def embed_key
+          if key = option(:embed_key)
+            key
+          else
+            :id
+          end
+        end
+
         def polymorphic_key
           associated_object.class.to_s.demodulize.underscore.to_sym
         end
@@ -191,15 +207,15 @@ module ActiveModel
             if associated_object
               {
                 :type => polymorphic_key,
-                :id => associated_object.read_attribute_for_serialization(:id)
+                :id => associated_object.read_attribute_for_serialization(embed_key)
               }
             else
               nil
             end
-          elsif source_serializer.object.respond_to?("#{name}_id")
+          elsif !option(:embed_key) && source_serializer.object.respond_to?("#{name}_id")
             source_serializer.object.send("#{name}_id")
           elsif associated_object
-            associated_object.read_attribute_for_serialization(:id)
+            associated_object.read_attribute_for_serialization(embed_key)
           else
             nil
           end

test/association_test.rb

@@ -34,12 +34,12 @@ class AssociationTest < ActiveModel::TestCase
     @root_hash = {}
 
     @post = Model.new(:title => "New Post", :body => "Body")
-    @comment = Model.new(:id => 1, :body => "ZOMG A COMMENT")
+    @comment = Model.new(:id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT")
     @post.comments = [ @comment ]
     @post.comment = @comment
 
     @comment_serializer_class = def_serializer do
-      attributes :id, :body
+      attributes :id, :external_id, :body
     end
 
     @post_serializer_class = def_serializer do
@@ -75,7 +75,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -105,7 +105,7 @@ class AssociationTest < ActiveModel::TestCase
       }, @hash)
 
       assert_equal({
-        :comments => [{ :id => 1, :body => "ZOMG A COMMENT" }]
+        :comments => [{ :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }]
       }, @root_hash)
     end
   end
@@ -124,7 +124,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -142,7 +142,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -160,7 +160,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -178,7 +178,79 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
+        ]
+      }, @root_hash)
+    end
+
+    def test_with_default_has_many_with_custom_embed_key
+      @post_serializer_class.class_eval do
+        has_many :comments, :embed_key => :external_id
+      end
+
+      include! :comments
+
+      assert_equal({
+        :comment_ids => [ "COMM001" ]
+      }, @hash)
+
+      assert_equal({
+        :comments => [
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
+        ]
+      }, @root_hash)
+    end
+
+    def test_with_default_has_one_with_custom_embed_key
+      @post_serializer_class.class_eval do
+        has_one :comment, :embed_key => :external_id
+      end
+
+      include! :comment
+
+      assert_equal({
+        :comment_id => "COMM001"
+      }, @hash)
+
+      assert_equal({
+        :comments => [
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
+        ]
+      }, @root_hash)
+    end
+
+    def test_with_default_has_many_with_custom_key_and_custom_embed_key
+      @post_serializer_class.class_eval do
+        has_many :comments, :key => :custom_comments, :embed_key => :external_id
+      end
+
+      include! :comments
+
+      assert_equal({
+        :custom_comments => [ "COMM001" ]
+      }, @hash)
+
+      assert_equal({
+        :comments => [
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
+        ]
+      }, @root_hash)
+    end
+
+    def test_with_default_has_one_with_custom_key_and_custom_embed_key
+      @post_serializer_class.class_eval do
+        has_one :comment, :key => :custom_comment, :embed_key => :external_id
+      end
+
+      include! :comment
+
+      assert_equal({
+        :custom_comment => "COMM001"
+      }, @hash)
+
+      assert_equal({
+        :comments => [
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -192,7 +264,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @hash)
 
@@ -237,7 +309,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -280,7 +352,7 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal({
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, @root_hash)
     end
@@ -336,7 +408,7 @@ class AssociationTest < ActiveModel::TestCase
           :comment_ids => [ 1 ]
         },
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, json)
     end
@@ -387,7 +459,7 @@ class AssociationTest < ActiveModel::TestCase
           :comment_ids => [ 1 ]
         },
         :comments => [
-          { :id => 1, :body => "ZOMG A COMMENT" }
+          { :id => 1, :external_id => "COMM001", :body => "ZOMG A COMMENT" }
         ]
       }, json)
     end