Delete logic
This is a WorkInProgress description of the NHProjectRepository logic in DS.
NHProjectRepository uses a custom method te determine which objects can be deleted from the database. Unfortunately NHibernate can not do this because NHibernate cascades are evaluated very locally. See for example http://fabiomaulo.blogspot.com/2009/09/nhibernate-tree-re-parenting.html Basically NHibernate can delete object B if object A is deleted but it cannot check if object B is in use elsewhere.
The solution:
S - P = O
SessionObjects - ProjectObjects are Orphans. So everything that is in session but not in project can be deleted. Now our save logic look liks this
1) Determine session objects using NHibernate session's entityentries
2) Determine project objects using EntityWalker (more about this later)
3) Determine orphaned objects by substracting 1 -2 and delete them from the session
4) Flush the session.
EntityWalker and EntityMetaModel
EntityWalker is a class that based on a EntityMetaModel can traverse a object tree. EntityMetaModel describes the relationship between objects and the strength of the relationship _(composition vs aggregation). The EntityMetaModel can be determined by looking at NHibernate's configuration and this is done by the NHibernateMetaModelProvider. Using this EMM the EntityWalker can create a set of objects which are reachable from the project root. The EntityWalker uses only Composite relationships to traverse the object tree. So something that is only reachable by aggregate relationships is not considered in project (and will be considered an orphan if is in session).
NHibernateMetaModelProvider
This class is responsible for creating a EntityMetaModel based on a NHibernate configuration. It uses NHibernate's cascade styles to determine the strength of the relationship. Here the rule of thumb is that when a cascade includes delete the relation is a composition and otherwise it is a mere aggregate relationship. So
- All -> Composite
- All-Delete-Orphan -> Composite
- Others -> Aggregation