Models can have validations preventing them from entering undesired conditions.

Two different types of constraints can be used:

• The ones checked at the database level
• The ones checked at the server level

Database-level constraints are limited to the constraints supported by PostgreSQL. The most commonly used ones are the UNIQUE constraints, but the CHECK and EXCLUDE constraints can also be used. If these are not enough for our needs, we can use Odoo server-level constraints written in Python code.

We will use the Library Book model created in Chapter 4, Creating Odoo Addon Modules, and add a couple of constraints to it. We will add a database constraint preventing duplicate book titles, and a Python model constraint preventing release dates in the future.