[Fixed]-Django One-To-One and One-To-Many Relations

You can use a simple CharField with choices to assign an entry to a specific container:

class Entry(models.Model):
    REFERENCE = 'reference'
    BACKBURNER = 'backburner-item'
    ACTION_STEP = 'action-step'
    CONTAINER_CHOICES = (
        (REFERENCE, 'Reference'),
        (BACKBURNER, 'Backburner item'),
        (ACTION_STEP, 'Action step'),
    )
    container = models.CharField(choices=CONTAINER_CHOICES)
    project = models.ForeignKey('Project', on_delete=models.CASCADE)
    ...

You can easily query for entries belonging to a specific container, either from a Project instance or from the Entry class:

references = project.entry_set.filter(container=Entry.REFERENCE)