We will add a new Python file, models/library_book_categ.py, for the category tree, as follows:

1. To load the new Python code file, add the following line to models/__init__.py:

from . import library_book_categ

2. To create the Book Category model with the parent and child relations, create the models/library_book_categ.py file with the following code:

from odoo import models, fields, api 
class BookCategory(models.Model): 
    _name = 'library.book.category' 
    name = fields.Char('Category') 
    parent_id = fields.Many2one( 
        'library.book.category', 
        string='Parent Category', 
        ondelete='restrict', 
        index=True) 
    child_ids = fields.One2many( 
        'library.book.category', 'parent_id', 
        string='Child Categories') 

3. To enable the special hierarchy support, also add the following code:

_parent_store = True
_parent_name = "parent_id" # optional if field is 'parent_id'
parent_path = fields.Char(index=True)

4. To add a check preventing looping relations, add the following line to the model:

from odoo.exceptions import ValidationError
...
@api.constrains('parent_id') 
def _check_hierarchy(self): 
    if not self._check_recursion(): 
        raise models.ValidationError( 
            'Error! You cannot create recursive categories.') 

5. Now, we need to assign a category to a book. To do this, we will add a new many2one field in the library.book model:

category_id = fields.Many2one('library.book.category')  

Finally, a module upgrade will make these changes effective.