How can I get the parent term in a taxonomy with a hierarchy?

Question

I have a node that is tagged with a taxonomy term which is part of a hierarchy of terms such as "state >> county."

As the user has selected one of the "county" tags, how can I get the name of the parent term?

Answer 1

You can query the database yourself, or you can use the API function: taxonomy_get_parents

Since you might have several parents, it might be easier to just query the database yourself:

$tid = ? // the tid of the county term
$parent_tid = db_result(db_query("SELECT parent FROM {term_hierarchy} WHERE tid = %d;",
  array(':tid' => $tid)
));
$parent_term = taxonomy_get_term($parent_tid);

Update - Why I think the above query is smart in Drupal 6

Some (kiamlaluno) would argue that it's wrong to use the above argument for db_query in Drupal 6

In Drupal 6 You can use db_query in three ways:

$query = db_query($sql, $arg1, $arg2, $arg3);
$query = db_query($sql, array($arg1, $arg2, $arg3));
$query = db_query($sql, array(':arg1' => $arg1, ':arg2' => $arg2, ':arg3' => $arg3));

All of the above will return the same thing, so there are no functional difference using one over the over. Why then you might ask, do I think it's clever to use the last one?

If you look at how Drupal 7 does it you will see:

$query = db_query($sql, array(':arg1' => $arg1, ':arg2' => $arg2, ':arg3' => $arg3));

Drupal 7 requires the query arguments formatted in a special way. What you can't see in this example is that the SQL needs to be changed as well. Instead of using placeholders like %d you need to use place holders like :arg1. So while the latter of the 3 Drupal 6 ways of using db_query is the most verbose, it will save you a lot of work, should you need to upgrade your Drupal 6 module to Drupal 7. Which might happen in a year, where you don't remember what you were doing and why.

But at the end of the day, it's just personal preference, as either way will work just fine.

Answer 2

taxonomy_get_parents() returns all the parent terms for the taxonomy term with the ID passed as argument.
The value returned from the function is an array having the following structure:

array(
  'term id' => [taxonomy term object]
)

The taxonomy term object contains a property for each of the fields contained in the "term_data" table.

The code used from the function is essentially the following one.

$result = db_query(db_rewrite_sql('SELECT t.tid, t.* FROM {term_data} t INNER JOIN {term_hierarchy} h ON h.parent = t.tid WHERE h.tid = %d ORDER BY weight, name', 't', 'tid'), $tid);
$parents = array();
while ($parent = db_fetch_object($result)) {
  $parents[$parent->$key] = $parent;
}

As you see, it doesn't return the parent terms of the parent terms of the one passed as argument. For that you need taxonomy_get_parents_all(), which executes the following code.

  if ($tid) {
    $parents[] = taxonomy_get_term($tid);
    $n = 0;
    while ($parent = taxonomy_get_parents($parents[$n]->tid)) {
      $parents = array_merge($parents, $parent);
      $n++;
    }
  }

The function returns all the parent terms in the hierarchy.