odoo - openerp中的父左和父右

Question

什么是父左和父右？

它在Openerp中如何工作？

谢谢

Answer 1

正如 Raphael Collet 在 his answer about OpenERP Server 中所解释的那样:

parent_left and parent_right are special fields that are related to the parent_id field. The purpose of those fields is to make queries within the hierarchy execute efficiently: with parent_left and parent_right, you can retrieve all the descendants of a node without making recursive queries.

Consider two nodes A and B in the hierarchy. A and B can be partner categories, for instance. Their integer fields parent_left and parent_right are such that:

    B is a descendant of A in the hierarchy (defined by parent_id)

if and only if

    A.parent_left < B.parent_left and
    A.parent_left < B.parent_right and 
    B.parent_left < A.parent_right and
    B.parent_right < A.parent_right

So, imagine that you have six partner categories like below. You can assign parent_left and parent_right by traversing the tree. The result is show in parentheses next to each node. Note that the values there are optimal; in practice, you can leave gaps in the numbers.

• Customers (1, 10)
  • Consumers (2, 3)
  • Partners (4, 9)
    • Basic Partners (5, 6)
    • Gold Partners (7, 8)
• Suppliers (11, 12)

You can retrieve all the subcategories of Customers with a single SQL query. Note that the values 1 and 10 are the parent_left and parent_right of Customers; they can be retrieved as part of the query itself.

    SELECT id FROM partner_category
    WHERE parent_left > 1 AND parent_left < 10

The last remark is that parent_left and parent_right can be updated without traversing the whole hierarchy. Removing a node does not require any change. For adding a node, you can adapt the parent_left and parent_right with two UPDATE queries: one to "make some space" between the parent_left and parent_right of the new node's ancestors, and one to shift the parent_left and parent_right of the new node's following siblings and their descendants. So parent_left and parent_right can be maintained efficiently.