add method to find existing_child_types, use default_child instead of…

… Page in case you want to override it

app/models/archive_page.rb

@@ -2,14 +2,16 @@ class ArchivePage < Page
   cattr_accessor :allowed_children
   cattr_accessor :single_use_children
   @@single_use_children = [ArchiveDayIndexPage, ArchiveMonthIndexPage, ArchiveYearIndexPage, FileNotFoundPage]
-  @@allowed_children = [Page, *@@single_use_children]
+  @@allowed_children = [self.new.default_child, *@@single_use_children]
 
   def allowed_children
-    allowed = @@allowed_children.dup
-    existing_types = children.all(:select => 'DISTINCT class_name, virtual, title').map(&:class_name)
-    overlap = allowed.map(&:to_s) & existing_types
+    overlap = @@allowed_children & (existing_child_types - [default_child])
 
-    allowed - overlap.map!(&:constantize)
+    (@@allowed_children - overlap)
+  end
+
+  def existing_child_types
+    children(:select => 'DISTINCT class_name', :order => nil).collect{|p| p.class }.uniq
   end
 
   description %{

spec/models/archive_page_spec.rb

@@ -32,13 +32,21 @@
   its(:single_use_children){ should == [ArchiveDayIndexPage, ArchiveMonthIndexPage, ArchiveYearIndexPage, FileNotFoundPage]}
   its(:allowed_children){ should == [Page, *ArchivePage.single_use_children]}
 
+  describe '#existing_child_types' do
+    it 'should return a unique array of classes of the page children' do
+      archive.existing_child_types.should == archive.children.all(:select => 'DISTINCT class_name').collect{|p| p.class }
+    end
+  end
+
   describe '#allowed_children' do
     context 'when no children exist' do
       subject{ ArchivePage.new }
       its(:allowed_children){ should == ArchivePage.allowed_children }
     end
     it 'should remove any existing single_use_children from the allowed_children' do
-      archive.allowed_children.should == [Page, FileNotFoundPage]
+      existing_except_default = archive.existing_child_types - [archive.default_child]
+      used_allowed_children = ArchivePage.allowed_children & existing_except_default
+      archive.allowed_children.should == (ArchivePage.allowed_children - used_allowed_children)
     end
   end
 end