preload associations if possible

lib/simple_form/form_builder.rb

@@ -156,7 +156,7 @@ module SimpleForm
         when :belongs_to
           reflection.options[:foreign_key] || :"#{reflection.name}_id"
         when :has_one
-          raise ":has_one association are not supported by f.association"
+          raise ":has_one associations are not supported by f.association"
         else
           if options[:as] == :select
             html_options = options[:input_html] ||= {}
@@ -167,6 +167,12 @@ module SimpleForm
           :"#{reflection.name.to_s.singularize}_ids"
       end
 
+      # Force the association to be preloaded for performance.
+      if options[:preload] != false && object.respond_to?(association)
+        target = object.send(association)
+        target.to_a if target.respond_to?(:to_a)
+      end
+
       input(attribute, options.merge(:reflection => reflection))
     end
 

test/form_builder_test.rb

@@ -585,6 +585,26 @@ class FormBuilderTest < ActionView::TestCase
     end
   end
 
+  test 'builder caches given association' do
+    value = @user.company
+    value.expects(:to_a).returns(value)
+    with_association_for @user, :company
+    assert_select 'form select.select#user_company_id'
+    assert_select 'form select option[value=1]', 'Company 1'
+    assert_select 'form select option[value=2]', 'Company 2'
+    assert_select 'form select option[value=3]', 'Company 3'
+  end
+
+  test 'builder does not cache given association if preload false' do
+    value = @user.company
+    value.expects(:to_a).never
+    with_association_for @user, :company, :preload => false
+    assert_select 'form select.select#user_company_id'
+    assert_select 'form select option[value=1]', 'Company 1'
+    assert_select 'form select option[value=2]', 'Company 2'
+    assert_select 'form select option[value=3]', 'Company 3'
+  end
+
   # ASSOCIATIONS - BELONGS TO
   test 'builder creates a select for belongs_to associations' do
     with_association_for @user, :company

test/test_helper.rb

@@ -66,7 +66,7 @@ class ActionView::TestCase
       :age => 19,
       :amount => 15,
       :attempts => 1,
-      :company => 1
+      :company => [1]
     }.merge(options))
 
     @other_validating_user = OtherValidatingUser.new({