Assigning bugs to the right components is the prerequisite to get the bugs analyzed and fixed. Classification-based techniques have been used in practice for assisting bug component assignments, for example, the BugBug tool developed by Mozilla. However, our study on 124,477 bugs in Mozilla products reveals that erroneous bug component assignments occur frequently and widely. Most errors are repeated errors and some errors are even misled by the BugBug tool. Our study reveals that complex component designs and misleading component names and bug report keywords confuse bug component assignment not only for bug reporters but also developers and even bug triaging tools. In this work, we propose a learning to rank framework that learns to assign components to bugs from correct, erroneous and irrelevant bug-component assignments in the history. To inform the learning, we construct a bug tossing knowledge graph which incorporates not only goal-oriented component tossing relationships but also rich information about component tossing community, component descriptions, and historical closed and tossed bugs, from which three categories and seven types of features for bug, component and bug-component relation can be derived. We evaluate our approach on a dataset of 98,587 closed bugs (including 29,100 tossed bugs) of 186 components in six Mozilla products. Our results show that our approach significantly improve bug component assignments for both tossed and non-tossed bugs over the BugBug tool and the BugBug tool enhanced with component tossing relationships, with >20% Top-k accuracies and >30% NDCG@k (k=1,3,5,10).