Migration Guide

From 2.x to 3.x

Defining abilities without a subject is not allowed anymore. For example, can :dashboard is not going to be accepted anymore and will raise an exception. All these kind of rules need to be rethought in terms of can action, subject. can :read, :dashboard for example.
Eager loading is not automatic. If you relied on CanCanCan to avoid N+1 queries, this will not be the case anymore. From now on, all necessary includes, preload or eager_load need to be explicitly written. We strongly suggest to have bullet gem installed to identify your possible N+1 issues.
Use of distinct. Uniqueness of the results is guaranteed by using the distinct clause in the final query. This may cause issues with some existing queries when using clauses like group by or order on associations. Adding a custom select may be necessary in these cases.
aliases are now merged. When using the method to merge different Ability files, the aliases are now also merged. This might cause some incompatibility issues.