How to Implement Field-Level Constraints in Odoo?

✅ What Are They?

Both are used to validate data and enforce business rules in Odoo, but they differ in where and how they work.

Feature	@api.constrains	SQL Constraints (_sql_constraints)
Type	Python-based ORM constraint	PostgreSQL-level constraint
Triggered On	create() and write()	Automatically at the DB level
Can Be Conditional	✅ Yes	❌ No (purely structural)
Supports Complex Logic	✅ Yes (if/else, cross-field validation)	❌ No
User Feedback	ValidationError in Python	DB-level error message

📌 Realistic Use Case 1: Prevent Duplicate Emails on Customers

Using SQL Constraint

_sql_constraints = [

    ('email_unique', 'unique(email)', 'Email must be unique.')

]

• Applied at the database level.
• Prevents duplicate values.
• Fast and efficient, but only for simple, single-field rules.

📌 Realistic Use Case 2: Ensure Discount ≤ 50% for Special Products

Using @api.constrains

• Triggers when discount or product_id is changed.
• Can use custom logic, conditions, and multiple fields.
• Ideal for business logic validations.
  
  

⚙️ When to Use What?

Scenario	Use @api.constrains	Use SQL Constraint
Cross-field validation	✅ Yes	❌ No
Simple uniqueness or non-null checks	❌ No	✅ Yes
Dynamic rules based on other fields	✅ Yes	❌ No
Performance-critical constraints	❌ Slower	✅ Faster
Needs translatable error messages	✅ Yes	✅ Yes

✅ Summary Table

Criteria	@api.constrains	SQL Constraint (_sql_constraints)
Level	Application / ORM	Database
Complex conditions	✅ Supported	❌ Not supported
Cross-field validation	✅ Yes	❌ No
Automatic enforcement	On create/write in ORM	On DB commit
Error type	ValidationError	DB constraint violation error
Customizable message	✅ Yes	✅ Yes

✅ Best Practices

• Use SQL constraints for structural validation (e.g., uniqueness).
• Use @api.constrains for business logic validation involving conditions or multiple fields.
• Avoid putting complex logic inside SQL constraints;