Hi,

In Odoo, a true one-to-one relationship isn’t available as a direct field type, but it can be simulated using a Many2one field combined with a unique SQL constraint. For example, if you want each customer (res.partner) to have exactly one passport record (res.passport), you can define a Many2one field in the passport model linking it to the partner and add a SQL constraint to ensure each partner can only have one passport. This setup effectively mimics a one-to-one relationship since no two passport records can reference the same partner.

To enforce uniqueness on any field, Odoo provides SQL constraints through the _sql_constraints attribute. By defining a rule such as ('unique_passport_number', 'unique(name)', 'Passport number must be unique!'), you ensure that duplicate entries for that field are not allowed in the database. Together, these techniques allow developers to maintain data integrity and simulate one-to-one relationships between models in Odoo Enterprise or Community.

Please refer to the following for more information.

Relational Fileds

* https://www.cybrosys.com/blog/what-is-the-difference-between-many2one-vs-one2many-in-odoo-18

* https://www.odoo.com/documentation/19.0/developer/tutorials/server_framework_101/07_relations.html

* https://www.cybrosys.com/odoo/odoo-books/odoo-17-development/creating-odoo-modules/relational-fields/

* https://www.cybrosys.com/odoo-development-tutorial/v17/many2one-in-odoo17/

Constraints

* https://www.cybrosys.com/odoo/odoo-books/odoo-17-development/creating-odoo-modules/constraints/

* https://www.cybrosys.com/odoo/videos/technical/how-to-use-api-constrains-in-odoo-17

* https://www.cybrosys.com/blog/constraints-in-odoo-16-erp

* https://www.odoo.com/documentation/19.0/developer/tutorials/server_framework_101/10_constraints.html

Hope it helps

Is we can't set-up even unique field constraints in odoo I have odoo enterprise plan hosted on odoo standard hosting, What i want is that instead of editing the base model like contacts i want to store many additional information of a contact, i create another model and create a relationship with contact model, but here the problem is i can't set-up one to one relationship, Only one to many, that's the issue, due to this I can't view contact basic details in my second model using related field 

2. My next question is to how can I Setup Unique  constraint on a. Field or something similar so when user enters the value and if the value exists in database it don't allow new creation of updation

Since you say "setup" and "configuration" - you can't do this in Odoo Studio - or with an Odoo Online Standard plan.

And in general: ORM of Odoo does not have a One2one relationship field type. What you usually would do is to create a Many2one on one model and a One2many relationship on the other model. Then, on that other model, you'd create a computed Many2one relationship back to the first model (<- this is the part you wouldn't be able to do in Studio).

When you want to develop in Odoo, you will not get around reading https://www.odoo.com/documentation/19.0/developer/reference/backend/orm.html as a whole.

Constrains are described here as well: https://www.odoo.com/documentation/19.0/developer/reference/backend/orm.html#constraints-and-indexes

class AModel(models.Model):
    _name = 'a.model'
    _my_check = models.Constraint("CHECK (x > y)", "x > y is not true")