title: Python Tortoise-ORM: use related model field in __str__
body: <p>I'm developing an API service with AIOHTTP, I try to integrate some async ORM, and the first candidate is Tortoise-ORM.
  In a Django project I have lots of linked models with <code>__str__</code> method like the following:</p>
  <pre class="lang-py prettyprint-override"><code>from tortoise.models import Model
  from tortoise import fields
  
  class Department(Model):
      id = fields.IntField(pk=True)
      title = fields.TextField()
      upper = fields.ForeignKeyField('models.Department', related_name='children')
  
      def __str__(self):
          if self.upper is not None:
              return f'{self.id} Department {self.title} of {self.upper.title}'
          else:
              return f'{self.id} Department {self.title}, head'
  
  class Employee(Model):
      id = fields.IntField(pk=True)
      name = fields.TextField()
      dep = fields.ForeignKeyField('models.Department', related_name='employees')
  
      def __str__(self):
          return f'{self.id}. Employee {self.name} of {self.dep.title}'
  </code></pre>
  <p>So that each object shows it's related models in description. But in Tortoise I get an error:</p>
  <blockquote>
  <p>AttributeError: 'QuerySet' object has no attribute 'title'</p>
  </blockquote>
  <p>I guess it's not possible to await the query in the <code>__str__</code> method. So, is it possible at all use a field of related model to create object representation using Tortoise-ORM?</p>