To enforce data integrity, models also support two types of constraints: SQL and Python

SQL constraints are added to the database table definition and are enforced directly by PostgreSQL. They are defined using the _sql_constraints class attribute. It is a list of tuples with: the constraint identifier name; the SQL for the constraint; and the error message to use.

A common use case is to add unique constraints to models. Suppose we don't want to allow two active tasks with the same title:

# class TodoTask(models.Model): 
    _sql_constraints = [ 
        ('todo_task_name_uniq', 
         'UNIQUE (name, active)', 
         'Task title must be unique!')] 

Python constraints can use a piece of arbitrary code to check the conditions. The checking function should be decorated with @api.constraints, indicating the list of fields involved in the check. The validation is triggered when any of them is modified and will raise an exception if the condition fails...