Are the Methods in Your Data Access Objects (DAOs) in the Right Place? A Preliminary Study

Isolating code that deals with system infrastructure from code that deals with domain rules is a good practice when developing applications. Code that deals with the database, for example, is often isolated in classes following a Data Access Object (DAO) pattern. Developers often create a DAO for each domain entity in the system. However, as some pieces of code deal with more than one entity/table, developers need to decide in which DAO they will place the code, and sometimes choose a less intuitive location. In this paper, we present a heuristic to identify methods that may have been written in an ambiguous place. To validate the idea, we tested it on three industrial projects from a Brazilian company. The heuristic selected, on average, 13% to 18% of the methods in DAOs. After evaluating such methods, we concluded that the heuristic was correct in 50% to 75% of cases. Therefore, we believe that the heuristic can indicate possible technical debt, where the developers may inspect and possibly refactor.