I am getting this error when trying to save a new record of the Estudiantes model, I am able to save if the carrera_ids field is empty but if I add one Carrera to it then I get the error

Integrity Error

The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: uccart.carreras - uccart.carreras]

These are my models

class Carreras (models.Model):
    _name = 'uccart.carreras'

    name = fields.Char(string="Carrera")
    codigo = fields.Char(string="Código")

    estudiante_ids   = fields.Many2many('uccart.estudiantes','uccart_carreras_estudiantes','estudiante','carrera')

class Carreras_Estudiantes(models.Model):
    _name = 'uccart.carreras.estudiantes'
    carrera    = fields.Many2one('uccart.carreras')
    estudiante = fields.Many2one('uccart.estudiantes')

class Estudiantes (models.Model):
    _name = 'uccart.estudiantes'
    _inherits = {'res.partner':'partner_id'}

    carrera_ids   = fields.Many2many('uccart.carreras','uccart_carreras_estudiantes','carrera','estudiante')

I have made other Many2many relationships before in tha same way, don't know what the error is, help is much appreciated