[celery#305]: Improving abstract models implementation.

Added a `helpers` module into `models` containing the functions
`taskresult_model()` and `groupresult_model()`.

  * `taskresult_model()`: will try to find the custom model using a
    dotted path defined under the constant
`CELERY_RESULTS_TASKRESULT_MODEL` in the settings of the user's project
  * `groupresult_model()` will try to do the same using under the
    constant `CELERY_RESULTS_GROUPRESULT_MODEL`.

By default if these attributes are not found `django-celery-results`
will use the default models (`models.TaskResult` and
`models.GroupResult`).

Updated database backend in order to use custom models for `TaskResult
and `GroupResult` it they're present.

Instead to import explicitely the `TaskResult` and the `GroupResult`
(default models from `django-celery-results`) we make use of the model
helpers to load the right classes, the custom ones if they're present
otherwise we use the default ones.

Getting data from `task_kwargs` to extend the `task_properties` and be
able to store them into the database (using the custom models).

First of all we need a way to get data from `task_kwargs` (or somewhere
else) just before a `task_result` record is created, evaluate that data
and find the right values that will be used to fill the new fields
defined in the custom model.

So for this purpose we added a settings module to
`django-celery-results` which will hold default settings, the first
setting that will contain is a function in charge to get a callback from
the settings of the user project. This callback will be feeded by the
task `task_kwargs`, which will be intercepted in
`DatabaseBackend._get_extended_properties` just before the `task_kwargs`
are encoded by `encode_content()` method and send it to the
`store_result` method from the object manager of `TaskModel`
(Custom/Default one).

To end, we must to extend the arguments of the `store_result` method
from the `TaskResult` Manager adding `**extra_fields` that will make us
able to send extra data to the custom model, when it's defined.

---
Resolves celery#305

Fixes celery#314

Author: diegocastrum

Diff for: .gitignore

@@ -30,3 +30,4 @@ cover/
 .cache/
 htmlcov/
 coverage.xml
+.vscode

Diff for: django_celery_results/admin.py

@@ -10,7 +10,10 @@
     ALLOW_EDITS = False
     pass
 
-from .models import GroupResult, TaskResult
+from .models.helpers import taskresult_model, groupresult_model
+
+GroupResult = groupresult_model()
+TaskResult = taskresult_model()
 
 
 class TaskResultAdmin(admin.ModelAdmin):

Diff for: django_celery_results/backends/database.py

@@ -12,8 +12,8 @@
 from kombu.exceptions import DecodeError
 
 from ..models import ChordCounter
-from ..models import GroupResult as GroupResultModel
-from ..models import TaskResult
+from ..models.helpers import taskresult_model, groupresult_model
+from ..settings import extend_task_props_callback
 
 EXCEPTIONS_TO_CATCH = (InterfaceError,)
 
@@ -29,8 +29,8 @@
 class DatabaseBackend(BaseDictBackend):
     """The Django database backend, using models to store task state."""
 
-    TaskModel = TaskResult
-    GroupModel = GroupResultModel
+    TaskModel = taskresult_model()
+    GroupModel = groupresult_model()
     subpolling_interval = 0.5
 
     def exception_safe_to_retry(self, exc):
@@ -79,6 +79,10 @@ def _get_extended_properties(self, request, traceback):
                 # task protocol 1
                 task_kwargs = getattr(request, 'kwargs', None)
 
+            # TODO: We assuming that task protocol 1 could be always in use. :/
+            extended_props.update(
+                extend_task_props_callback(getattr(request, 'kwargs', None)))
+
             # Encode input arguments
             if task_args is not None:
                 _, _, task_args = self.encode_content(task_args)

Diff for: django_celery_results/models/managers.py renamed to django_celery_results/managers.py

@@ -8,7 +8,7 @@
 from django.conf import settings
 from django.db import connections, models, router, transaction
 
-from ..utils import now, raw_delete
+from .utils import now, raw_delete
 
 W_ISOLATION_REP = """
 Polling results with transaction isolation level 'repeatable-read'
@@ -121,7 +121,7 @@ def store_result(self, content_type, content_encoding,
                      traceback=None, meta=None,
                      periodic_task_name=None,
                      task_name=None, task_args=None, task_kwargs=None,
-                     worker=None, using=None):
+                     worker=None, using=None, **extra_fields):
         """Store the result and status of a task.
 
         Arguments:
@@ -143,6 +143,7 @@ def store_result(self, content_type, content_encoding,
                 exception (only passed if the task failed).
             meta (str): Serialized result meta data (this contains e.g.
                 children).
+            **extra_fields (dict): Extra (model)fields to store.
 
         Keyword Arguments:
         -----------------
@@ -163,7 +164,8 @@ def store_result(self, content_type, content_encoding,
             'task_name': task_name,
             'task_args': task_args,
             'task_kwargs': task_kwargs,
-            'worker': worker
+            'worker': worker,
+            **extra_fields
         }
         obj, created = self.using(using).get_or_create(task_id=task_id,
                                                        defaults=fields)

Diff for: django_celery_results/models/abstract.py

@@ -5,7 +5,7 @@
 from django.db import models
 from django.utils.translation import gettext_lazy as _
 
-from . import managers
+from .. import managers
 
 ALL_STATES = sorted(states.ALL_STATES)
 TASK_STATE_CHOICES = sorted(zip(ALL_STATES, ALL_STATES))

Diff for: django_celery_results/models/generic.py

@@ -21,6 +21,7 @@ class Meta(AbstractTaskResult.Meta):
         """Table information."""
 
         abstract = False
+        app_label = "django_celery_results"
 
 
 class ChordCounter(models.Model):
@@ -48,6 +49,10 @@ class ChordCounter(models.Model):
         )
     )
 
+    class Meta:
+        app_label = "django_celery_results"
+
+
     def group_result(self, app=None):
         """Return the GroupResult of self.
 
@@ -71,3 +76,4 @@ class Meta(AbstractGroupResult.Meta):
         """Table information."""
 
         abstract = False
+        app_label = "django_celery_results"

Diff for: django_celery_results/models/helpers.py

@@ -0,0 +1,47 @@
+from django.apps import apps
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+
+from .generic import TaskResult, GroupResult
+
+def taskresult_model():
+    """Return the TaskResult model that is active in this project."""
+    if not hasattr(settings, 'CELERY_RESULTS_TASKRESULT_MODEL'):
+        return TaskResult
+    
+    try:
+        return apps.get_model(
+            settings.CELERY_RESULTS_TASKRESULT_MODEL
+        )
+    except ValueError:
+        raise ImproperlyConfigured(
+            "CELERY_RESULTS_TASKRESULT_MODEL must be of the form "
+            "'app_label.model_name'"
+        )
+    except LookupError:
+        raise ImproperlyConfigured(
+            "CELERY_RESULTS_TASKRESULT_MODEL refers to model "
+            f"'{settings.CELERY_RESULTS_TASKRESULT_MODEL}' that has not "
+            "been installed"
+        )
+
+def groupresult_model():
+    """Return the GroupResult model that is active in this project."""
+    if not hasattr(settings, 'CELERY_RESULTS_GROUPRESULT_MODEL'):
+        return GroupResult
+    
+    try:
+        return apps.get_model(
+            settings.CELERY_RESULTS_GROUPRESULT_MODEL
+        )
+    except ValueError:
+        raise ImproperlyConfigured(
+            "CELERY_RESULTS_GROUPRESULT_MODEL must be of the form "
+            "'app_label.model_name'"
+        )
+    except LookupError:
+        raise ImproperlyConfigured(
+            "CELERY_RESULTS_GROUPRESULT_MODEL refers to model "
+            f"'{settings.CELERY_RESULTS_GROUPRESULT_MODEL}' that has not "
+            "been installed"
+        )

Diff for: django_celery_results/settings.py

@@ -0,0 +1,16 @@
+from django.conf import settings
+
+
+def get_callback_function(settings_name, default=None):
+    """Return the callback function for the given settings name."""
+    
+    callback = getattr(settings, settings_name, None)
+    if callback is None:
+        return default
+
+    if callable(callback):
+        return callback
+
+extend_task_props_callback = get_callback_function(
+    "CELERY_RESULTS_EXTEND_TASK_PROPS_CALLBACK"
+)