Raise error when associations cannot be included

include! only works when the source serializer has a root set. The
as_json method sets up some state for the include! method. If a child
association has associations with `:include => true` or `root foo,
:include => true` would cause an undefined method error for `NilClass`.
This is entirely unhelpful for the end user.

This commit raise an error when this situation occurs. It makes it clear
that it's not a problem with AMS but the serialization graph.

lib/active_model/serializer.rb

@@ -108,6 +108,18 @@ module ActiveModel
   #     end
   #
   class Serializer
+    class IncludeError < StandardError
+      attr_reader :source, :association
+
+      def initialize(source, association)
+        @source, @association = source, association
+      end
+
+      def to_s
+        "Cannot serialize #{association} when #{source} does not have a root!"
+      end
+    end
+
     module Associations #:nodoc:
       class Config #:nodoc:
         class_attribute :options
@@ -502,7 +514,9 @@ module ActiveModel
       if association.embed_ids?
         node[association.key] = association.serialize_ids
 
-        if association.embed_in_root?
+        if association.embed_in_root? && hash.nil?
+          raise IncludeError.new(self.class, association.name)
+        elsif association.embed_in_root?
           merge_association hash, association.root, association.serialize_many, unique_values
         end
       elsif association.embed_objects?

test/serializer_test.rb

@@ -1169,4 +1169,79 @@ class SerializerTest < ActiveModel::TestCase
       }
     }, actual)
   end
+
+  def test_raises_an_error_when_a_child_serializer_includes_associations_when_the_source_doesnt
+    attachment_serializer = Class.new(ActiveModel::Serializer) do
+      attributes :name
+    end
+
+    fruit_serializer = Class.new(ActiveModel::Serializer) do
+      embed :ids, :include => true
+      has_one :attachment, :serializer => attachment_serializer
+      attribute :color
+    end
+
+    banana_class = Class.new Model do
+      def self.to_s
+        'banana'
+      end
+
+      def attachment
+        @attributes[:attachment]
+      end
+
+      define_method :active_model_serializer do
+        fruit_serializer
+      end
+    end
+
+    strawberry_class = Class.new Model do
+      def self.to_s
+        'strawberry'
+      end
+
+      def attachment
+        @attributes[:attachment]
+      end
+
+      define_method :active_model_serializer do
+        fruit_serializer
+      end
+    end
+
+    smoothie = Class.new do
+      attr_reader :base, :flavor
+
+      def initialize(base, flavor)
+        @base, @flavor = base, flavor
+      end
+    end
+
+    smoothie_serializer = Class.new(ActiveModel::Serializer) do
+      root false
+      embed :ids, :include => true
+
+      has_one :base, :polymorphic => true
+      has_one :flavor, :polymorphic => true
+    end
+
+    banana_attachment = Attachment.new({
+      :name => 'banana_blending.md',
+      :id => 3,
+    })
+
+    strawberry_attachment = Attachment.new({
+      :name => 'strawberry_cleaning.doc',
+      :id => 4
+    })
+
+    banana = banana_class.new :color => "yellow", :id => 1, :attachment => banana_attachment
+    strawberry = strawberry_class.new :color => "red", :id => 2, :attachment => strawberry_attachment
+
+    smoothie = smoothie_serializer.new(smoothie.new(banana, strawberry))
+
+    assert_raise ActiveModel::Serializer::IncludeError do
+      smoothie.as_json
+    end
+  end
 end