Addition of new property to the UmbracoChildren attribute

Addition of new property to the UmbracoChildren attribute to allow mapping of different node types as children.

When you have different types of nodes as children in Umbraco, the mapper needs to know which document type the collection maps to.

I have added a new property to the attribute - DocumentTypeAlias - which allows the property to determine which type it maps to. This allows filtering of the nodes when fetched from the CMS. An exception is thrown otherwise, because all child nodes are added to the collection, regardless of the type.

There may be a cleaner way to achieve this, by avoiding all the nasty if-then nesting.

Thanks,

Dan.