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Hi all:

I just found out that the Google Rich Snippets Testing tool rejects all RDFa markup for products if the data contains foaf:page (a link to the HTML page) with a value that is NOT from within the domain name space of the actual file location.

Example:

If you have the following markup in the page available from http://www.domain.com/example.html, then

a) an empty foaf:page value will work

 <div typeof="gr:Offering" about="#offering" >
      <div property="gr:name" content="ACME Anvil" xml:lang="en"></div>
      <div rel="foaf:page" resource=""></div>

b) foaf:page with the actual page URI will work

 <div typeof="gr:Offering" about="#offering" >
      <div property="gr:name" content="ACME Anvil" xml:lang="en"></div>
      <div rel="foaf:page" resource="http://www.domain.com/example.html"></div>

c) foaf:page with a URI from the same domain name space as the actual page URI will also work:

 <div typeof="gr:Offering" about="#offering" >
      <div property="gr:name" content="ACME Anvil" xml:lang="en"></div>
      <div rel="foaf:page" resource="http://www.domain.com/canonical/example.html"></div>

BUT:

c) foaf:page with a URI from another domain name space than the actual page URI will NOT work:

 <div typeof="gr:Offering" about="#offering" >
      <div property="gr:name" content="ACME Anvil" xml:lang="en"></div>
      <div rel="foaf:page" resource="http://www.some_other_domain.com/example.html"></div>


Unfortunately, the testing tool gives the very uninformative error message

"Insufficient data to generate the preview.
... 
Warning: In order to generate a preview, either price or review or availability needs to be present."

It is natural that Google does not like foaf:page pointers to external domain name spaces, but this unexpected behavior costed me almost a day.

So watch out!

Option b) is the recommended practice, because it works in any scenario, but it requires that you know the URI of the final publishing destination at markup-time (or the template has access to the deep URI as a variable).

Option a) is handier, because the URI of the current page will be automatically used as the value for foaf:page, but it 
fails, if the document sets the base URI of all product item pages to the main shop page, e.g. via

	<base href="http://shop.com/home/" />

because then all offers would point to the same base page and deep links would be lost.

Best wishes

Martin Hepp