It looks like I cannot create a new record in the stock_availables via API when it does not exist, I am getting an error
<error> <code><![CDATA[101]]></code> <message><![CDATA[Method POST is not allowed for the resource stock_availables]]></message> </error>
when my product looks like this
<?xml version="1.0" encoding="UTF-8"?> <prestashop xmlns:xlink="http://www.w3.org/1999/xlink"> <product> <id><![CDATA[79]]></id> ... <stock_availables nodeType="stock_available" api="stock_availables"/> <accessories nodeType="product" api="products"/> <product_bundle nodeType="product" api="products"/> </associations> </product> </prestashop>
I believe associations between product and stock_availables dissapeared after I added this line while updating products
$clientXml->children()[0]->associations->categories->category[0]->id=$idCategory;
because products without associated categories did not appear in the shop. Now products do appear in the shop, but I cannot update the qty in stock anymore 
.
I noticed I also have 59 orphan records in the ps_stock_available table for not existing products. SQL I used to find orphans:
SELECT pa.* FROM `ps_stock_available` pa WHERE not exists (select p.id_product from ps_product p where p.id_product=pa.id_product)
Any ideas how to fix it?
PS version 1.7.5.2
Well, my problem was id_shop_group was set to 1 in my code, not specifying the category. Sorry.