id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26827	"""ModelState.fields cannot refer to a model class ... Use a string reference instead."" when using custom model field derived from ManyToMany"	Rich Rauenzahn	Prashant Pandey	"I ended up in a weird situation (Django 1.9.7) where I couldn't make any additional migrations.  So I reset my migrations thinking the problem was somehow in the migration scripts themselves:


{{{
    ValueError: ModelState.fields cannot refer to a model class - ""runs.to"" does. Use a string reference instead.
}}}

I still got the error after restarting migrations from scratch, so I determined it is about the migration looking at the currently declared models, not the historical model migration scripts.  

I have this (trimmed) code:


{{{
class Run(models.Model):
       pass

class _RunsField(models.ManyToManyField):                                       
                                                                                  
      def __init__(self, *args, **kwargs):                                        
          super(_RunsField, self).__init__(Run, through='BaselineAssignment')   
                                                                                  
                                                                                  
class Baseline(models.Model):
                                                                                                                                                                   
      runs = _RunsField()

 class BaselineAssignment(models.Model):               
      run = models.ForeignKey('Run', on_delete=models.CASCADE)                    
      baseline = models.ForeignKey('Baseline', on_delete=models.CASCADE)          
 

}}}

If I change {{{Run}}} in the {{{__init__}}} in {{{_RunsField}}} to {{{'Run'}}} (quoted) the problem goes away.

This seems like this is something that should have been handled by the underlying migration code, and even if it isn't, the error message is quite unhelpful."	Cleanup/optimization	closed	Documentation	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0