• Parent reference of assignment gets lost on move.
• Probably related to inverse property parent to assignments of AssignmentCollection.
• Does removing parent column in assignment solve the problem?
• How can loop in assignment tree be created?

I finished an example exam which can be used by the students to practice with WebLab. However, when reorganising the questions, some of them are no longer reachable from the top-level. All questions in http://department.st.ewi.tudelft.nl/weblab/assignment/728 should be part of http://department.st.ewi.tudelft.nl/weblab/assignment/729, but I do not see any way to move them.

Manifestation: https://yellowgrass.org/issue/WebLab/19

Submitted by Eelco Visser on 13 March 2013 at 14:12

Initial suspicion was that this was related to evolution of the database representation of the parent-assignments relation. Turns out to be an ordinary bug caused by an incompletely executed pull-up method refactoring of the toNextSibling method.

Tip: including a validate statement in the method to check the expected property of the objects at the end of the method was useful to confirm the problem. Useful to include in more places to verify invariants.