diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 26e0a7f9c2..d2cfdc6a4d 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Assigning nil to a composed_of aggregate also sets its immediate value to nil.  #9843 [Chris Cruft]
+
 * Ensure that mysql quotes table names with database names correctly.  Closes #9911 [crayz]
 
   "foo.bar" => "`foo`.`bar`"  
diff --git a/activerecord/lib/active_record/aggregations.rb b/activerecord/lib/active_record/aggregations.rb
index 8c3cdc3c11..8ee9d8368d 100644
--- a/activerecord/lib/active_record/aggregations.rb
+++ b/activerecord/lib/active_record/aggregations.rb
@@ -148,7 +148,7 @@ def reader_method(name, class_name, mapping, allow_nil)
           mapping = (Array === mapping.first ? mapping : [ mapping ])
 
           allow_nil_condition = if allow_nil
-            mapping.collect { |pair| "!read_attribute(\"#{pair.first}\").nil?"}.join(" && ")
+            mapping.collect { |pair| "!read_attribute(\"#{pair.first}\").nil?"}.join(" || ")
           else
             "true"
           end
@@ -169,10 +169,10 @@ def writer_method(name, class_name, mapping, allow_nil)
           if allow_nil
             module_eval <<-end_eval, __FILE__, __LINE__
               def #{name}=(part)
+                @#{name} = part.freeze
                 if part.nil?
                   #{mapping.collect { |pair| "@attributes[\"#{pair.first}\"] = nil" }.join("\n")}
                 else
-                  @#{name} = part.freeze
                   #{mapping.collect { |pair| "@attributes[\"#{pair.first}\"] = part.#{pair.last}" }.join("\n")}
                 end
               end
diff --git a/activerecord/test/aggregations_test.rb b/activerecord/test/aggregations_test.rb
index 8cd4bfe481..89927e5044 100644
--- a/activerecord/test/aggregations_test.rb
+++ b/activerecord/test/aggregations_test.rb
@@ -92,4 +92,19 @@ def test_allow_nil_address_set_to_nil
   def test_nil_raises_error_when_allow_nil_is_false
     assert_raises(NoMethodError) { customers(:david).balance = nil }
   end
+
+  def test_allow_nil_address_loaded_when_only_some_attributes_are_nil
+    customers(:zaphod).address_street = nil
+    customers(:zaphod).save
+    customers(:zaphod).reload
+    assert_kind_of Address, customers(:zaphod).address
+    assert customers(:zaphod).address.street.nil?
+  end
+
+  def test_nil_assignment_results_in_nil
+    customers(:david).gps_location = GpsLocation.new('39x111')
+    assert_not_equal nil, customers(:david).gps_location
+    customers(:david).gps_location = nil
+    assert_equal nil, customers(:david).gps_location
+  end
 end