WooCommerce: issue setting attributes with wp_set_object_terms

I need to set product's attributes via PHP code. According to other threads on StackOverflow, there's the method wp_set_object_terms. Unfortunately it is not working as it is supposed to do in my case:

For example, there is an attribute named "Hersteller", slug "hersteller". And there is a product, ID 593, with no "hersteller" attribute set.

I tried the following code to fill the attribute:

wp_set_object_terms(593, 'Alpina', 'pa_hersteller' , false);

When I'm attempting to display the attribute on the product page, there is no output, so it seems that the wp_set_object_terms process hasn't been successful. Following code produces an empty output:

$product->get_attribute('hersteller');

Furthermore, the attribute "hersteller" isn't even listed in the attribute list in the admin backend menue.

To debug the problem I've also tried the following code:

$attributes = get_post_meta( 593,  '_product_attributes' ); 
print_r($attributes);

resulting in the following output:

Array ( [0] => Array ( [pa_marken] => Array ( [name] => pa_marken [value] => [position] => 0 [is_visible] => 1 [is_variation] => 0 [is_taxonomy] => 1 ) [pa_kategorien] => Array ( [name] => pa_kategorien [value] => [position] => 1 [is_visible] => 1 [is_variation] => 0 [is_taxonomy] => 1 ) [pa_referenznummer] => Array ( [name] => pa_referenznummer [value] => [position] => 2 [is_visible] => 1 [is_variation] => 0 [is_taxonomy] => 1 ) [pa_herstellergarantie] => Array ( [name] => pa_herstellergarantie [value] => [position] => 3 [is_visible] => 1 [is_variation] => 0 [is_taxonomy] => 1 ) [pa_schlagwoerter] => Array ( [name] => pa_schlagwoerter [value] => [position] => 4 [is_visible] => 1 [is_variation] => 0 [is_taxonomy] => 1 ) [pa_lieferzeit] => Array ( [name] => pa_lieferzeit [value] => [position] => 5 [is_visible] => 1 [is_variation] => 0 [is_taxonomy] => 1 ) ) )

Am I using the wp_set_object_terms method essentially wrong? I don't know what to do anymore now. May anyone here can help me? Thanks!

via Chebli Mohamed