[xwiki-users] Assign blog category through groovy

Hi I need to assign a blog category to my document created by code? def vCategory = "Informatie" vObjArticle.set("category",vCategory) category is a special typed-attribute in BlogPostClass object of type "database tree" I have defined the blog category "Informatie" through the category editor of the blog application but after saving the document and inspecting the object it does not show the tekst "Informatie", but it appears to be empty Is this the right way of setting such a attribute? If not, how to? Gerritjan