Start cleaning up serializers to support

include/exclude functionality.

This process involves building a few lower-level
primitives so not as much logic is hardcoded in
monolithic helper methods.

test/association_test.rb

@@ -0,0 +1,67 @@
+require "test_helper"
+
+class SerializerTest < ActiveModel::TestCase
+  def def_serializer(&block)
+    Class.new(ActiveModel::Serializer, &block)
+  end
+
+  class Model
+    def initialize(hash={})
+      @attributes = hash
+    end
+
+    def read_attribute_for_serialization(name)
+      @attributes[name]
+    end
+
+    def as_json(*)
+      { :model => "Model" }
+    end
+
+    def method_missing(meth, *args)
+      if meth.to_s =~ /^(.*)=$/
+        @attributes[$1.to_sym] = args[0]
+      elsif @attributes.key?(meth)
+        @attributes[meth]
+      else
+        super
+      end
+    end
+  end
+
+  def test_include_associations
+    post = Model.new(:title => "New Post", :body => "Body")
+    comment = Model.new(:id => 1, :body => "ZOMG A COMMENT")
+    post.comments = [ comment ]
+
+    comment_serializer_class = def_serializer do
+      attributes :body
+    end
+
+    post_serializer_class = def_serializer do
+      attributes :title, :body
+    end
+
+    post_serializer = post_serializer_class.new(post, nil)
+
+    hash = {}
+    root_hash = {}
+    post_serializer.include! :comments,
+      :embed => :ids,
+      :include => true,
+      :hash => root_hash,
+      :node => hash,
+      :value => post.comments,
+      :serializer => comment_serializer_class
+
+    assert_equal({
+      :comments => [ 1 ]
+    }, hash)
+
+    assert_equal({
+      :comments => [
+        { :body => "ZOMG A COMMENT" }
+      ]
+    }, root_hash)
+  end
+end