diff --git a/lib/dm-core/model.rb b/lib/dm-core/model.rb
index d4e377e70..3e7a8e787 100644
--- a/lib/dm-core/model.rb
+++ b/lib/dm-core/model.rb
@@ -209,11 +209,12 @@ def self.extra_extensions
     def self.extended(descendant)
       descendants << descendant
 
-      descendant.instance_variable_set(:@valid,         false)
-      descendant.instance_variable_set(:@base_model,    descendant)
-      descendant.instance_variable_set(:@storage_names, {})
-      descendant.instance_variable_set(:@default_order, {})
-
+      descendant.instance_eval do
+        @valid           = false unless instance_variable_defined?(:@valid)
+        @base_model    ||= descendant
+        @storage_names ||= {}
+        @default_order ||= {}
+      end
       descendant.extend(Chainable)
 
       extra_extensions.each { |mod| descendant.extend(mod)         }
@@ -224,6 +225,11 @@ def self.extended(descendant)
     def inherited(descendant)
       descendants << descendant
 
+      descendant.instance_eval do
+        @valid        = false unless instance_variable_defined?(:@valid)
+        @base_model ||= base_model
+      end
+
       descendant.instance_variable_set(:@valid,         false)
       descendant.instance_variable_set(:@base_model,    base_model)
       descendant.instance_variable_set(:@storage_names, @storage_names.dup)
diff --git a/lib/dm-core/model/property.rb b/lib/dm-core/model/property.rb
index 09fa15aee..fcc692675 100644
--- a/lib/dm-core/model/property.rb
+++ b/lib/dm-core/model/property.rb
@@ -7,13 +7,17 @@ module Property
       Model.append_extensions self, DataMapper::Property::Lookup
 
       def self.extended(model)
-        model.instance_variable_set(:@properties,               {})
-        model.instance_variable_set(:@field_naming_conventions, {})
+        model.instance_eval do
+          @properties                 = {}
+          @field_naming_conventions ||= {}
+        end
       end
 
 
       def inherited(model)
-        model.instance_variable_set(:@properties,               {})
+        model.instance_eval do
+          @properties ||= {}
+        end
         model.instance_variable_set(:@field_naming_conventions, @field_naming_conventions.dup)
 
         @properties.each do |repository_name, properties|
diff --git a/lib/dm-core/model/relationship.rb b/lib/dm-core/model/relationship.rb
index aeb5380ae..ec1c2cd15 100644
--- a/lib/dm-core/model/relationship.rb
+++ b/lib/dm-core/model/relationship.rb
@@ -17,7 +17,9 @@ module Relationship
       #
       # @api private
       def self.extended(model)
-        model.instance_variable_set(:@relationships, {})
+        model.instance_eval do
+          @relationships ||= {}
+        end
       end
 
       # When DataMapper model is inherited, relationships
@@ -25,7 +27,9 @@ def self.extended(model)
       #
       # @api private
       def inherited(model)
-        model.instance_variable_set(:@relationships, {})
+        model.instance_eval do
+          @relationships ||= {}
+        end
 
         @relationships.each do |repository_name, relationships|
           model_relationships = model.relationships(repository_name)
diff --git a/spec/public/model/relationship_spec.rb b/spec/public/model/relationship_spec.rb
index 2cacfc57b..ae5eaaf2c 100644
--- a/spec/public/model/relationship_spec.rb
+++ b/spec/public/model/relationship_spec.rb
@@ -1038,3 +1038,64 @@ class ::Company
     end
   end
 end
+
+describe DataMapper::Associations do
+  before :all do
+    module ::Vehicle
+      def self.included(base)
+        base.extend(ClassMethods)
+      end
+
+      module ClassMethods
+
+        def vehicle_with_wheels(*args)
+          include DataMapper::Resource
+
+          property :id, DataMapper::Property::Serial
+          property :license_plate, String
+        end
+      end
+    end
+
+    class Exhaust
+      include DataMapper::Resource
+
+      property :id, Serial
+      belongs_to :car
+    end
+
+    class Suspension
+      include DataMapper::Resource
+
+      property :id, Serial
+      property :brand, String
+    end
+
+    class Car
+      include DataMapper::Resource
+      has n, :suspensions
+    end
+
+    class Car
+      include DataMapper::Resource
+      include Vehicle
+
+      vehicle_with_wheels
+
+      property :number_of_seats, Integer
+      has n, :exhausts
+
+    end
+
+    DataMapper.finalize
+  end
+
+  it "should have all relationships when Resource is included once" do
+    Exhaust.relationships.any? {|r| r.name == :car }.should be_true
+  end
+
+  it "should not forget about relationships when Resource is included more than once" do
+    Car.relationships.any? {|r| r.name == :exhausts }.should be_true
+    Car.relationships.any? {|r| r.name == :suspensions }.should be_true
+  end
+end