TypeError: get_sort_key() missing 1 required positional argument: 'ctx'
peewee==3.5.2
Python 3.6.5 Oh, the model contained a bad attribute:
```python
    assigned_at = peewee.DateTimeField
```
2.10.2 detected this, while 3.5.2 showed this error.
After I added the parens, it worked.

Should I close this issue? > 2.10.2 detected this, while 3.5.2 showed this error.

What did 2.10.2 do when this happened? Something like "Unknown field". I think I'm going to leave this as-is for now, as I'm not sure of a clean way to implement this. Very unintuitive error message. This issue probably saved me hours of debugging. I did the same thing @mindojo-victor did. Maybe something in `BaseModel.__new__` like this:
```
for field in dir(cls):
    if field.startswith('__'):
        pass
    if type(getattr(cls, field)) == type:
        raise ValueError('Fields must be initialized.')
```

I could work on PR Whoo that was a fun 2 hrs trying to figure out what was wrong  -- I was missing the parens after IntegerField

Is there an easier way to detect this issue in the future?
What this might mean? I have no idea how to fix this. Or is it a bug in the `peewee` code?
```python
  File "/Users/Vic/projects/brain2/brain/utils/reports/bi.py", line 449, in sync_author_tasks
    BIAuthorTask.insert_many(author_tasks).on_conflict_replace().execute()
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 1560, in inner
    return method(self, database, *args, **kwargs)
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 1631, in execute
    return self._execute(database)
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 2274, in _execute
    return super(Insert, self)._execute(database)
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 2049, in _execute
    cursor = database.execute(self)
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 2637, in execute
    sql, params = ctx.sql(query).query()
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 549, in sql
    return obj.__sql__(self)
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 2258, in __sql__
    self._generate_insert(self._insert, ctx)
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 2181, in _generate_insert
    columns = sorted(accum, key=lambda obj: obj.get_sort_key(ctx))
  File "/Users/Vic/projects/brain2/venv/lib/python3.6/site-packages/peewee.py", line 2181, in <lambda>
    columns = sorted(accum, key=lambda obj: obj.get_sort_key(ctx))
TypeError: get_sort_key() missing 1 required positional argument: 'ctx'
```