“…Despite of its success, collaborative filtering approaches are known to suffer from data sparsity issues, as the number of items or users is typically very large but the number of ratings is relatively small. One popular way to address this issue is to incorporate the increasingly available side information in the model [2,5,15,28,29,31]. The majority of studies exploit either only flat side information [2,5], or only hierarchical side information [15,31] due to the challenges brought by the inherent difference between these two types of information.…”