Nested sets or Nested Set Model is a way to effectively store hierarchical data in a relational table. From wikipedia:
The nested set model is to number the nodes according to a tree traversal, which visits each node twice, assigning numbers in the order of visiting, and at both visits. This leaves two numbers for each node, which are stored as two attributes. Querying becomes inexpensive: hierarchy membership can be tested by comparing these numbers. Updating requires renumbering and is therefore expensive.
NSM shows good performance when tree is updated rarely. It is tuned to be fast for getting related nodes. It’is ideally suited for building multi-depth menu or categories for shop.
Suppose that we have a model Category; a $node variable is an instance of that model and the node that we are manipulating. It can be a fresh model or one from database.
Node has following relationships that are fully functional and can be eagerly loaded:
- Node belongs to
parent - Node has many
children - Node has many
ancestors - Node has many
descendants
