"Relationship:" dropdown isn't built correctly

The views_ui_config_item_form() function (in admin.inc) is responsible for constructing the "Relationship:" dropdown that's shown, when applicable, on every field form.

However, that function checks for applicability thus:

$base_fields = views_fetch_fields($base, $form_state['type']);
if (isset($base_fields[$handler->table . '.' . $handler->field])) {

Instead of thus:

$base_fields = views_fetch_fields($base, $form_state['type']);
if (isset($base_fields[$item['table'] . '.' . $item['field']])) {

While doing $handler->table . '.' . $handler->field is fine for most cases, there's one instance where it fails: a relationship handler may overwrite $this->field by $this->definition['relationship field']. So doing $handler->table . '.' . $handler->field in this case will fail to find this relationship in any table.

(This can be seen in a bug report in the Flag module's queue.)

The "relationship field" definition is quite handy (it's like the "real field" definition), so this bug might bother many more people in the future.

(Incidentally, the same problem happens with the "relationship table" definition, but I didn't mention it because it'd be rarer to use this definition. The patch fixes this case as well.)

The patch fixes this by switching to $item['table'] . '.' . $item['field'].