'Tries to re-evaluate the _failed_fields for each model.'

def retry_failed_fields(self):

    for (modelkey, model) in self.models.items():
        (app, modelname) = modelkey.split('.', 1)
        if hasattr(model, '_failed_fields'):
            for (fname, (code, extra_imports)) in model._failed_fields.items():
                try:
                    field = self.eval_in_context(code, app, extra_imports)
                except (NameError, AttributeError, AssertionError, KeyError) as e:
                    raise ValueError(("Cannot    successfully    create    field    '%s'    for    model    '%s':    %s." % (fname, modelname, e)))
                else:
                    model.add_to_class(fname, field)
