setCategoryIds() changes product visibility to "Catalog,Search"

Question

Can somebody explain why setCategoryIds($ids) changes the product's visibility to Catalog,Search from whatever else it was (in my encounter, I was working with Search-view products)?

Here's the code that makes all of the products in $categoryId to have visibility Catalog,Search after the save() when they originally had Search. I restored all of my products and tested it to verify as well.

$productCollection = Mage::getModel('catalog/category')->load($categoryId)
    ->getProductCollection() 
    ->addAttributeToSelect(array('name','price','special_price','sku','small_image'));

foreach($productCollection as $product) {
    $catIds = $product->getCategoryIds();
    $product->setCategoryIds($catIds);
    $product->save();
}

Answer 1

The reason is that you're saving the product but you've not loaded the visibility as part of the product collection. Catalog, Search is the default visibility.

Try this:

$productCollection = Mage::getModel('catalog/category')->load($categoryId)
    ->getProductCollection() 
    ->addAttributeToSelect(array('name','price','special_price','sku','small_image'))
    ->joinAttribute('visibility', 'catalog_product/visibility', 'entity_id', null, 'inner');

Now when you save the product it should retain its original visibility setting.