Django query on related model - mysql

Question

I have models like below
class Scheduler(models.Model):
id = <this is primary key>
last_run = <referencing to id in RunLogs below>
class RunLogs(models.Model):
id = <primary key>
scheduler = <referencing to id in Scheduler above>
overall_status = <String>
Only when the scheduler reaches the scheduled time of the job, RunLogs entry is created.
Now I am querying on RunLogs to show running schedules as below.
current = RunLog.objects\
.filter(Q(overall_status__in = ("RUNNING", "ON-HOLD", "QUEUED") |
Q(scheduler__last_run__isnull = True))
The above query gives me all records with matching status from RunLogs but does not give me records from Scheduler with last_run is null.
I understand why the query is behaving so but is there a way to get records from scheduler also with last_run is null
?

Answer 1

I just did the same steps which you followed and found the reason why you where getting all the records after running your query. Here is the exact steps and a solution for this.
Steps
Created models
from django.db import models
class ResourceLog(models.Model):
id = models.BigIntegerField(primary_key=True)
resource_mgmt = models.ForeignKey('ResourceMgmt', on_delete=models.DO_NOTHING,
related_name='cpe_log_resource_mgmt')
overall_status = models.CharField(max_length=8, blank=True, null=True)
class ResourceMgmt(models.Model):
id = models.BigIntegerField(primary_key=True)
last_run = models.ForeignKey(ResourceLog, on_delete=models.DO_NOTHING, blank=True, null=True)
Added the data as following:
resource_log
+----+----------------+------------------+
| id | overall_status | resource_mgmt_id |
+----+----------------+------------------+
| 1 | RUNNING | 1 |
| 2 | QUEUED | 1 |
| 3 | QUEUED | 1 |
+----+----------------+------------------+
resource_mgmt
+----+-------------+
| id | last_run_id |
+----+-------------+
| 1 | NULL |
| 2 | NULL |
| 3 | NULL |
| 4 | 3 |
+----+-------------+
According to the above table resource_mgmt(4) is referring to resource_log(3). But thing to be noted is, resource_log(3) is not referring to resource_mgmt(4).
Ran the following command in python shell
In [1]: resource_log1 = ResourceLog.objects.get(id=1)
In [2]: resource_log.resource_mgmt
Out[2]: <ResourceMgmt: ResourceMgmt object (1)>
In [3]: resource_log1 = ResourceLog.objects.get(id=2)
In [4]: resource_log.resource_mgmt
Out[4]: <ResourceMgmt: ResourceMgmt object (1)
In [5]: resource_log1 = ResourceLog.objects.get(id=3)
In [6]: resource_log.resource_mgmt
Out[6]: <ResourceMgmt: ResourceMgmt object (1)>
from this we can understand that all the resource_log objects are referring to 1st object of resource_mgmt(ie, id=1).
Q) Why all the objects are referring to 1st object in the resource_mgmt?
resource_mgmt is a foreign key field which is not null. Its default value is 1. when you create a resource_log object, if you are not specifying resource_mgmt, it will add the default value there which is 1.
Run your query
In [60]: ResourceLog.objects.filter(resource_mgmt__last_run__isnull = True)
Out[60]: <QuerySet [<ResourceLog: ResourceLog object (1)>, <ResourceLog: ResourceLog object (2)>, <ResourceLog: ResourceLog object (3)>]>
This query is returning all three ResourceLog objects because all three are referring to 1st resource_mgmt object which has its is_null value as True
Solution
You actually want to check the reverse relationship.
We can achieve this using two queries:
rm_ids = ResourceMgmt.objects.exclude(last_run=None).values_list('last_run', flat=True)
current = ResourceLog.objects.filter(overall_status__in = ("RUNNING", "QUEUED")).exclude(id__in=rm)
The output is:
<QuerySet [<ResourceLog: ResourceLog object (1)>, <ResourceLog: ResourceLog object (2)>]>
Hope that helps!