Instance in Django model with problem

Question

I’m working on a multi-app project. The department app model is as follows::

from emails.models import GerenciarEmails

class Departamento(models.Model):
#modelo de grupos de disciplinas
class Meta:
    verbose_name = 'Departamento'
    verbose_name_plural = 'Departamentos'
titulo = models.CharField(max_length=255, verbose_name='Nome do Grupo')
watermark = models.FileField(upload_to = settings.MEDIA_ROOT+"/watermarks/", default=settings.MEDIA_ROOT+'/img/generico.gif')
cor = ColorField(blank=True)
gerencia = models.ForeignKey(GerenciarEmails)
docentes_responsaveis = models.ManyToManyField(settings.AUTH_USER_MODEL, null=True, blank=True,related_name="docentes_responsaveis_pelo_grupo", limit_choices_to={'docente':True})
def __unicode__(self):
    #Modo como o Modelo e Exibido na Lista na Adminstracao
    return unicode(self.titulo)

I added the following instance to it:

gerencia = models.ForeignKey(GerenciarEmails)

Mail management is a method of the email app model. The import what matters is this used, the others are related to other things.

With the insertion of this instance, Django gives the following error: Import: cannot import name Department

Apparently he is not letting enter this instance. Could someone help me on the reason for the error?

This is the method Managerial:

class GerenciarEmails(models.Model):
    class Meta:
        verbose_name = 'Gerenciar E-mails'
        verbose_name_plural = 'Gerenciar E-mails'
    departamento = models.ForeignKey(Departamento)
    def __unicode__(self):
        return self.departamento.titulo

As you can see, I use the Department method, from models.py from the Departments app. If I put import after this method, as is the suggestion from mgibsonbr, python gives another error:

ImportError: cannot import name Disciplina

It kind of makes sense, because Managerial needs the import of Department.

Answer 1

First thing is to check the folder structure of your project, maybe you have to put

from seuprojeto.emails.models import GerenciarEmails

If this is not the case, it is probably the problem that mgibsonbr said in the comments: "If your app A depends on (imports) B, and B in turn depends on A". Use Foreignkey this way to kill the problem:

gerencia = models.ForeignKey('emails.GerenciarEmails')

Answer 2

Your error message is an indication of circular dependency. You wouldn’t happen to have, in emails/models.py, a line making:

from departamentos.models import Departamento

? That code worked before, because the app emails depended on departamentos, then when emails/models.py was loaded, she did the import, loaded the app departamentos entire, and then continued to read the content of the app emails.

When you did that departamentos raisin also depending on emails (by the way, what is causing the error is the import, not the creation of the field), so we have a problem:

• Python starts to read departamentos/models.py; creates the module, empty;
• He reads the line he asks to import emails.models; the module has not yet been read;
• He starts to read emails/models.py; creates the module, empty;
• He reads the line he asks to import departamentos.models; the module exists, but is empty;
• He tries to read Departamento in the imported module; that name does not exist as the module is empty;
• ImportError: cannot import name Departamento

Solving this is not easy, and it may involve refactoring your code (changing app templates). If you had certainty that the app emails to be loaded before the app departamentos, you could put the import not at the beginning of the file, but after the creation of the template GerenciarEmails:

emails/models.py

... # Nada que faça uso de Departamento

class GerenciarEmails(models.Model):
    ...

from departamentos.models import Departamento

... # Coisas que dependem de Departamento

Thus, the app emails would be read to the point where it defines GerenciarEmails, with the import app departamentos would be read in full (and she would have access to GerenciarEmails, since it has already been included in the module) and then the emails/models.py would continue your reading normally. It works, the hard thing is to ensure that reading order (if the departamentos was read first, it’s over, there’s no way to "fix" the order).

In the reply from Puam Dias it suggests specifying the foreign key using a string, rather than a direct reference to a model:

gerencia = models.ForeignKey('emails.GerenciarEmails')

If you do this, no matter the app emails, can work (I can’t say, because I’ve never used this construction). If nothing else works, then all you have to do is move your templates from one app to another until there are no more circular dependencies between apps.