Dear Silviaa,

          According my knowledge , One2many is a virtual field.Since it is It must have a Many2one model behind.So that it,s data can be linked to the current record on the self.

for eg:

class Target(models.Model):
    _name = 'projection.target'
    _rec_name = 'sales_man'
    _sql_constraints = [('sales_man', 'UNIQUE(sales_man)', 'The employee has already assigned to a target'), ]
    sales_man = fields.Many2one('res.users', string="Select Employee", required=True)
    working_days = fields.Integer(compute="computewkng")
    target = fields.Char(string='Total Projection', readonly=True, compute='totalprojection')
    sal = fields.Float(string='Achievement', compute="calculatetotalsales", readonly=True)
    average = fields.Float(String='Average', compute='computeavg')
    variance = fields.Float(string='Variance', compute='computevariance')
    data = fields.One2many('projection.onetomanydata', 'inv', ondelete='cascade')
 #this model has a One2many field that is linked with the Many2one on another module
class OneToManyData(models.Model):
    _name = 'projection.onetomanydata'
    _rec_name = 'name'
    _sql_constraints = [('date', 'UNIQUE(date)', 'Employee Has Already Assigned to a Target on This date'), ]
    name = fields.Char(default='Daily Projection', invisible=True)
    date = fields.Date(string="Date", required=True)
    target = fields.Integer(string="Daily Projection", required=True)
    inv = fields.Many2one('projection.target')
#here the inv links this model to the first one
#its just a matter of 2 lines of code.