from django.db import models
    
    class ForceDefaultDBManager(models.Manager):
        db = 'default'  # Put your db alias here
        use_for_related_fields = True
    
        def using(self, *args, **kwargs):
            return self.get_query_set().using(self.db)
    
    class Library(models.Model):
        name = models.CharField()
    
    class Book(models.Model):
        library = models.ForeignKey(Library, related_name='books')
        name = models.CharField()
    
        objects = ForceDefaultDBManager()