Fix preloading of belongs_to with null foreign key generating useless query [#1027 state:resolved]

activerecord/lib/active_record/association_preload.rb

@@ -307,6 +307,7 @@ def preload_belongs_to_association(records, reflection, preload_options={})
 
         klasses_and_ids.each do |klass_and_id|
           klass_name, id_map = *klass_and_id
+          next if id_map.empty?
           klass = klass_name.constantize
 
           table_name = klass.quoted_table_name

activerecord/test/cases/associations/eager_test.rb

@@ -145,7 +145,7 @@ def test_finding_with_includes_on_belongs_to_association_with_same_include_inclu
   def test_finding_with_includes_on_null_belongs_to_association_with_same_include_includes_only_once
     post = posts(:welcome)
     post.update_attributes!(:author => nil)
-    post = assert_queries(2) { Post.find(post.id, :include => {:author_with_address => :author_address}) } # find the post, then find the author which is null so no query for the address
+    post = assert_queries(1) { Post.find(post.id, :include => {:author_with_address => :author_address}) } # find the post, then find the author which is null so no query for the author or address
     assert_no_queries do
       assert_equal nil, post.author_with_address
     end
@@ -705,4 +705,5 @@ def test_conditions_on_join_table_with_include_and_limit
   def test_order_on_join_table_with_include_and_limit
     assert_equal 5, Developer.find(:all, :include => 'projects', :order => 'developers_projects.joined_on DESC', :limit => 5).size
   end
+
 end