When the user is creating a new record in a model(for example creating a new contact) I want to check the uniqueness of a field value just at the moment the field is changed, not after finishing and saving the entire form.

Here is my sample code:

quotation_id = fields.Many2one(
	​comodel_name="sale.order",
	​string="Quotation ID",
	​required=True,
	​readonly=True
)
and this is my XML:

	​	​<field name="quotation_id"/>

I've defined a sql constraint as follow:

	​_sql_constraints = [
     	​       ('quotation_id_uniq', 'UNIQUE (quotation_id)', 'This Quotation Number is asigned to a previous spot. Please choose another one.')        
	​]

The user can enter an ID in the field. 
I want to check this field just as it changes and inform the user if ID is selected before.

How can I do that?