Currently, the celery executor imports the `Task` model for the Celery database backend directly:

https://github.com/apache/airflow/blob/2dd4e96045d4a7f45cc8c06df3d25c4f1479392c/airflow/executors/celery_executor.py#L38

However, Celery, being highly customisable, uses `self.task_cls` in the backend implementation; this defaults to either `Task` or `TaskExtended` depending on the Celery configuration, see [`celery.backends.database:66`](https://github.com/celery/celery/blob/406f04a082949ac42ec7a4af94fed896c515aaa4/celery/backends/database/__init__.py#L66):

> ```python
>     task_cls = Task
> ```

and [`celery.backends.database:76,77`](https://github.com/celery/celery/blob/406f04a082949ac42ec7a4af94fed896c515aaa4/celery/backends/database/__init__.py#L76-L77):

> ```python
>         if self.extended_result:
>             self.task_cls = TaskExtended
> ```

Airflow should support this flexibility, and instead of importing a fixed class just use `backend.task_cls`, so in `_get_many_from_db_backend` just use:

```python
        Task = app.backend.task_cls
        with session_cleanup(session):
            tasks = session.query(Task).filter(Task.task_id.in_(task_ids)).all()
```

I suppose there is one downside to this: `TaskExtended` fetches a few more columns per task, but given that the result of `task.to_dict()` is passed to `backend.meta_from_decoded()`, which could also have been customised to make use of information contained in a custom model, that's neither here nor there. It would be the price anyone using the `result_extended` option would need to pay elsewhere anyway.
airflow/executors/celery_executor.py
#
#
#
"""CeleryExecutor
.. seealso::
    For more information on how the CeleryExecutor works, take a look at the guide:
    :ref:`executor:CeleryExecutor`
"""
import datetime
import logging
import math
import operator
import os
import subprocess
import time
import traceback
from collections import OrderedDict
from multiprocessing import Pool, cpu_count
from typing import Any, Dict, List, Mapping, MutableMapping, Optional, Set, Tuple, Union
from celery import Celery, Task, states as celery_states
from celery.backends.base import BaseKeyValueStoreBackend
from celery.backends.database import DatabaseBackend, Task as TaskDb, session_cleanup
from celery.result import AsyncResult
from celery.signals import import_modules as celery_import_modules
from setproctitle import setproctitle  
import airflow.settings as settings
from airflow.config_templates.default_celery import DEFAULT_CELERY_CONFIG
from airflow.configuration import conf
from airflow.exceptions import AirflowException, AirflowTaskTimeout
from airflow.executors.base_executor import BaseExecutor, CommandType, EventBufferValueType