Drupal 7 Module Development

Chapter 6
Master/slave database configurations are an advanced topic that we
won't cover in detail. However, Drupal transparently falls back to
single-database behavior if no slave is defined so it's always good to
plan ahead for supporting a master/slave configuration.
Once again there are multiple points where we call the Fields system to let it
do its part with our entity, which again should be self-explanatory. We also call
module_invoke_all() in order to allow other modules to interact with any entity
when they are created or updated.
Handling revisions
The only other complex part of the process involves revision handling, which can be a
bit hard to wrap our head around. Looking at the saveRevision() method may help:
function saveRevision($artwork, $uid, $update = FALSE) {
// Hold on to the artwork's original creator_uid but swap
// in the revision's creator_uid for the momentary write.
$temp_uid = $artwork->uid;
$artwork->uid = $uid;
}
if ($update) {
drupal_write_record('artwork_revision', $artwork, 'vid');
}
else {
drupal_write_record('artwork_revision', $artwork);
}
// Reset the order's creator_uid to the original value.
$artwork->uid = $temp_uid;
After we've saved the artwork to the artwork table, we also save it to the
artwork_revision table. If we're creating a new revision, we'll save a new record to
the table and drupal_write_record() will populate the vid property for us with
the new version ID. Then, back in save(), we update the record in the artwork table
to point to the new revision record. If we just overwrote the old revision record then
that step is not necessary.
That's it! We can now create new artwork objects straight from the UI with a simple
form, including whatever Fields we've decided to attach to our artworks. Not only
that, but since we put the saving logic in the controller rather than in the form submit
callback we can easily create new artworks programmatically; we need only create a
new artwork object with the properties we want and call artwork_save().
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