"ModelState.fields cannot refer to a model class ... Use a string reference instead." when using custom model field derived from ManyToMany

[Rich Rauenzahn]

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 _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.