Handling Validation for Required Datetime Fields in Odoo:

In Odoo development, it's common to validate user input during form interactions. However, it's important to choose the right method for validation, especially when working with required fields such as datetime. A common mistake is using UserError in onchange methods for validation, which can lead to unexpected behavior.

The Problem with UserError in onchange

When a UserError is raised inside an onchange method for a required datetime field, the expected behavior is that the invalid or selected value gets cleared. However, this doesn't happen. Instead, the values remain in the form, which can confuse users.

This occurs because raising a UserError in an onchange doesn't prevent users from editing the form—it only shows an error message and halts further logic. It doesn't reset or clear fields, nor does it block saving the record unless explicitly checked elsewhere.

The Right Way: Use @api.constrains for Validation

To properly enforce validation, especially on required fields, it's better to use the @api.constrains decorator. When a UserError is raised in a constraint method:

• The record is not saved.

• The form retains its state, allowing users to correct input.

• Validation happens at the persistence layer, ensuring consistency across UI and backend logic.

Why This Works Better

• Constraints are triggered only on save, ensuring that the business logic is enforced reliably.

• User feedback remains consistent and clear.

• Required fields behave correctly, without residual values that confuse the user.

Conclusion

Use onchange for dynamic field updates and client-side adjustments. Use @constrains for validation that must be enforced before saving data. Avoid using UserError in onchange for validation purposes—this ensures a better user experience and cleaner code.