add has_one to active_hash.

lib/associations/associations.rb

@@ -50,6 +50,22 @@ def has_many(association_id, options = {})
         end
       end
 
+      def has_one(association_id, options = {})
+        define_method(association_id) do
+          options = {
+            :class_name => association_id.to_s.classify,
+            :foreign_key => self.class.to_s.foreign_key
+          }.merge(options)
+
+          scope = options[:class_name].constantize
+
+          if scope.respond_to?(:scoped) && options[:conditions]
+            scope = scope.scoped(:conditions => options[:conditions])
+          end
+          scope.send("find_by_#{options[:foreign_key]}", id)
+        end
+      end
+
       def belongs_to(association_id, options = {})
 
         options = {

spec/associations/associations_spec.rb

@@ -209,7 +209,43 @@ class Book < ActiveRecord::Base
         author.city_id.should == @city.id
       end
     end
+  end
+
+  describe "#has_one" do
+    context "with ActiveRecord children" do
+      before do
+        Author.has_one :book
+      end
+
+      it "find the correct records" do
+        book = Book.create! :author_id => 1, :published => true
+        author = Author.create :id => 1
+        author.book.should == book
+      end
+
+      it "returns nil when there is no record" do
+        author = Author.create :id => 1
+        author.book.should be_nil
+      end
+    end
 
+    context "with ActiveHash children" do
+      before do
+        City.has_one :author
+        Author.field :city_id
+      end
+
+      it "find the correct records" do
+        city = City.create :id => 1
+        author = Author.create :city_id => 1
+        city.author.should == author
+      end
+
+      it "returns nil when there are no records" do
+        city = City.create :id => 1
+        city.author.should be_nil
+      end
+    end
   end
 
   describe "#marked_for_destruction?" do