Recently a lot of NHibernate errors have come up. A very common cause is the cascade rules in the mapping as they require close attention. An example of a 'wrong' mapping was the mapping of composite branch structures (CS) (the mapping is shown below). The CS has a list of child structures and these were persisted using a cascade='all'. The same structures were also in the list of branchfeatures on the branch and these were mapped using cascade=all-delete-orphan. So the same weir was saved via branch and via the composite structure. This is fine as long as the cascade don't conflict.
Sometimes a conflict occurs when one cascade results in a delete and another in a save. For example a compositie structure is deleted and the cascade deletes all child-structures. But one of these structures is still in the branchfeatures collection of a branch. The branch cascade insists on the object being saved (or at least not deleted) and the CS cascade wants to delete. Hence a conflict. This can be fixed by removing the structure from both lists or downgrading/removing one of the cascades. In this case the branch should be responsible for saving the features and the CS should have a list of child structures without cascades.
