On 8/3/07, King Simon-NFHD78 <[EMAIL PROTECTED]> wrote: > I think of adjacency lists and nested sets as more about hierarchies of > a single type of object (imagine trying to represent a family tree with > a row for each Person). I don't really think they're relevant in this > case.
If I understand correctly the OP has a need to store a set of hierarchical "Nodes". Only that some node types (video, image) can't have children while others can (group, media_list). Alexandre, why do you find that the nested set model suits your needs better? The nested set model is particularly suited to the following type of queries: 1. find all children of a particular node, without needing to know their generation 2. test if node A is a descendant of node B These are queries that are expensive with adjacency lists but inexpensive with nested sets. Nested sets however may require you to update a lot of nodes if your tree changes. Traditionally you would do that with triggers or stored procedures - but those don't play well with ORMs which assumes that an update of a row won't invalidate cached instances of other rows. Do you have specific needs for those kind of queries? I don't know your application but I'd probably go with adjacency lists and a common "Node" superclass that provides access to child and parent nodes. Derive other classes from that and use joined table polymorphism as Simon suggested. There is another way to store hierarchical data than adjacency lists and nested sets though. If you have an invariant id on your objects (numerical or textual) you can store an objects "path", governed by object.path = object.parent.path + delimiter + object.id This might not be efficient for deeply nested trees maybe, but it can help in answering the types of queries mentioned above efficiently without requiring you to touch multiple rows upon inserting or deleting. Also, it would be easy to implement in SA and you could have all sorts of convenience methods on a Node class, like get_direct_children get_all_descendants is_parent is_child is_descendant Arnar --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~----------~----~----~----~------~----~------~--~---
