Allow for RDFa datatype attribute without callback function

This patch adds an RDF_PASSTHROUGH_CALLBACK flag to rdf.module. This patch also moves the datatype insertion out of the callback condition since it is independent from whether or not we want to use a content attribute. Here is an example of how to use the flag with the amount of comments for a node:

        'comment_count' => array(
          'predicates' => array('sioc:num_replies'),
          'datatype' => 'xsd:integer',
          'callback' => RDF_PASSTHROUGH_CALLBACK,
        ),

This is the output
<td class="replies" property="sioc:num_replies" content="0" datatype="xsd:integer">0</td>

Is it the syntax 'callback' => RDF_PASSTHROUGH_CALLBACK, that you find odd? Regardless we need to have a way to add a content attribute without having a callback function, this is a bug.

Alternative 1:
Change the signature of rdf_rdfa_attributes() and have:

function rdf_rdfa_attributes($mapping, $data = NULL, $passthrough = NULL) {

Alternative 2:
Define a real passthrough callback function like:

function rdf_passthrough($data) {
  return $data;
}

and use it in the mapping definition

        'comment_count' => array(
          'predicates' => array('sioc:num_replies'),
          'datatype' => 'xsd:integer',
          'callback' => 'rdf_passthrough',
        ),

patch for alternative 2 above. Less code changes and less special casing as Dries said. will update #614508: Annotate tracker page with RDFa to make use of this patch.

@stefan: how do you deal with the case in rdf_preprocess_field() where we always pass a $data value to rdf_rdfa_attributes() because we rely on the RDF mapping definition to know what to do with it?

        $variables['item_attributes_array'][$delta] = rdf_rdfa_attributes($mapping[$field_name], $item['#item']);

In this case we don't want the value to appear in a content attribute.

To summarize, here are the 3 cases we need to cover in RDFa:

1. value same as the one in the HTML tag

<a property="dc:title" href="/node/146">A node title</a>

In this snippet, the value for dc:title is simply the content of the a tag and we don't need to use a content attribute. Note that rdf_preprocess_field() unconditionally calls rdf_rdfa_attributes() and relies on the RDF mapping definition to figure out whether we need a content attribute or not (depending on whether there is a callback function or not).

2. value different from the one in the HTML tag

<td datatype="xsd:dateTime" content="2009-11-27T17:39:25-06:00" property="sioc:last_reply_date">1 day 20 hours ago</td>

In this snippet, we need to convert a date to the proper iso 8601 format using the date_iso8601 callback function.

3. value different from the one in the HTML tag, but no conversion needed (case currently missing)

<td datatype="xsd:integer" content="21" property="sioc:num_replies" class="replies">21<br/><a href="/node/149#new">21 new</a></td>

In this snippet, we do not need to convert the number of comments, but cannot use the content of the td tag because it contains extra code which would interfere with the value 21, so it's better to use a content attribute with the value 21. We suggest to specify a passthrough callback function in the RDF mapping definition.

same patch as in #4 since Stefan also agreed it's the best approach.

test galore!

That's a side effect of the updated test_entity mapping definition. updated rdf.test.

rerolling patch

now the testing bot is back, reverting to RTBC per #17.

setting back to RTBC since the testing bot has given the green light.

While working on #614508: Annotate tracker page with RDFa, we realized that the rdf_passthrough approach should not be integrated in rdf_rdfa_attributes() because the case we tried to cover is dependent on the theme layer and should not be handled at the RDF mapping level. It is up to the theme layer to add the appropriate content attribute if necessary, depending on what it outputs in the HTML tag. (see example at #614508-43: Annotate tracker page with RDFa).

This makes this issue much more simplier! Here is the new description which summarizes what the attached patch does (changing the title to reflect this).

Description

#614508: Annotate tracker page with RDFa revealed a bug in rdf_rdfa_attributes() which currently requires a callback function to be defined in the RDF mapping in order to output a datatype attribute in RDFa. However, as illustrated in the case 3 below, there are cases where we want to output a datatype without outputting a content attribute (and instead use the value of the HTML tag). The patch attached improves the logic of rdf_rdfa_attributes to support this use case. It also extends the tests to cover the various types of mapping like ObjectProperty and inverse ObjectProperty.

To summarize, here are the 3 cases we need to cover in RDFa:

1. Same value as the one in the HTML tag (no callback function)

<a property="dc:title" href="/node/146">A node title</a>

In this snippet, the value for dc:title is simply the content of the a tag and we don't need to use a content attribute.

2. Value different from the one in the HTML tag (callback function)

<td datatype="xsd:dateTime" content="2009-11-27T17:39:25-06:00" property="sioc:last_reply_date">1 day 20 hours ago</td>

In this snippet, the date was converted to the proper iso8601 format using the date_iso8601 callback function. The iso8601 date value appears in the content attribute.

3. Same value as the one in the HTML tag with datatype. (case currently missing)

<td datatype="xsd:integer" property="sioc:num_replies">21</td>

In this snippet, we do not need to convert the number (21) but we want to specify its datatype with datatype="xsd:integer".

The following snippet is not handled by rdf_rdfa_attributes and should be handled at the theme layer:

<td datatype="xsd:integer" content="21" property="sioc:num_replies">21<br/><a href="/node/149#new">21 new</a></td>

In this snippet, we do not need to convert the number of comments, but cannot rely on the value of the td tag because it contains extra code which would interfere with the value 21. Here it is recommended to use a content attribute with the value 21. rdf_rdfa_attributes() does not support this particular case: the content attribute should be added while building the HTML output, see an example in the tracker module and #614508: Annotate tracker page with RDFa.

this patch includes a small API fix so it's best to get it in before alpha.

I think Sunday has passed!