[ZODB-Dev] Re: Navigating inherently hierarchical data

[Tommy Li]

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          tseaver at palladion.com
> Palladion Software   "Excellence by Design"    http://palladion.com
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