Tres Seaver wrote:
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Tommy Li wrote:
Hi,
I'm just learning about Object databases, and I stumbled across Zope.
I'm trying to store data that inherently has a hierarchical (tree)
structure. As I have it currently implemented, each object points to
another object of the same type by storing the ID of its parent.
So currently in SQL, to find all the children of object id=4, I'd query:
SELECT * from TreeNodes WHERE parent=4;
To find the parent of a given object, obviously, I'd just query (say 3
is parent of 4):
SELECT * from TreeNodes WHERE id=3;
The tree structure is a very important part of the data and cannot be
compromised.
In the Wikipedia article describing object databases, it mentioned that
data access can be faster because related data could be found by
following pointers. This makes sense. In the object domain, if I
maintain a set of references for the parent and child in each TreeNode,
traversal around the tree would be very quick.
I don't understand how I can use ZODB to do this, however. From what I
can gather from reading the manual, if I simply stored variables
referring to the parent and children in every TreeNode, I'd end up
storing the whole tree of TreeNodes's every time I wanted to store that
one TreeNode. (Presumably, because ZODB would follow all the references
recursively.)
ZODB breaks reference cycles like that for persistent objects. You can
store child nodes as attributes of the parents, using Python's dot operator:
parent = Node('parent')
parent.one = Node('one')
parent.two = Node('two')
If the children need backpointers to the parent, you could pass the
parent to their constructor, or set a '__parent__' attribute on them:
parent.one.__parent__ = parent
parent.three = ChildNode('three', parent)
In Zope2, we have traditionally *not* set a backpointer, but relied on
the fact that getting the attribute from parent gives us back an
acquisition wrapper, which can then be used to find the parent (via
'aq_parent'). Zope3 has chosen a more explicit model for most objects,
with a '__parent__' reference.
I suppose I could use the old method of storing only the identifiers of
the parents/children. Is there a way ZODB can help me with efficient
traversal of persistent objects in a tree structure?
Under the covers, ZODB *does* mutate the objects to store "stubs" for
attributes whire are persistent: the stub consists of a reference to
the object's class, plus its OID. When you try to *use* the stub, ZODB
transparently replaces it with an object constructed from the state
stored in the ZODB for that OID.
Great. So when I store the parent node of a tree structure into zodb,
all the children get recursively stored?
Or do I need to manually store each of these Persistent subobjects? Is
there a specific order I need to store the parent and child in?
Tres.
- --
===================================================================
Tres Seaver +1 540-429-0999 [EMAIL PROTECTED]
Palladion Software "Excellence by Design" http://palladion.com
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