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EntityWalker is a class that based on a EntityMetaModel can traverse a object tree. EnityMetaModel describes the relationship between objects and the quality 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 EntityWalker uses only Composite relationship 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).