How to filter foreign key objects in django queries?

Question

I have two models, one related to other by foreign key like this

class CapturedPrescriptionModel(ColModel):
    p_id = models.IntegerField()
    p_age = models.IntegerField()
    p_gender = models.CharField(max_length=10)
    p_care_type = models.CharField(max_length=100)
    bacteria_id = models.ForeignKey(BacteriaListModel, 
              on_delete=models.CASCADE, null=True)

class SuggestedAntibioticsModel(ColModel):
    prescription_id = models.ForeignKey(CapturedPrescriptionModel, 
                  related_name='antibiotics', 
                   on_delete=models.CASCADE)
    cat_ids = models.TextField()
    flag = models.IntegerField(default=0)

Now I want all the prescriptions with suggested antibiotics where flag=1

I have tried with CapturedPrescriptionModel.objects.filter(antibiotics__flag=1) but that filter the prescriptions not the list of antibiotics in the queryset.

 [
    {
    "id": 7,
    "p_id": 0,
    "p_age": 19,
    "p_gender": "Male",
    "p_care_type": "ICU",
    "bacteria_id": null,
    "antibiotics": [
        {
            "id": 188,
            "cat_ids": "[]",
            "flag": 0,
            "antibiotic_id_id": 87,
            "prescription_id_id": 7
        },
        {
            "id": 187,
            "cat_ids": "[]",
            "flag": 1,
            "antibiotic_id_id": 112,
            "prescription_id_id": 7
        },
      ......
      ]
}
....
]

My expected result will be like this

    [
        {
        "id": 7,
        "p_id": 0,
        "p_age": 19,
        "p_gender": "Male",
        "p_care_type": "ICU",
        "bacteria_id": null,
        "antibiotics": [
            {
                "id": 187,
                "cat_ids": "[]",
                "flag": 1,
                "antibiotic_id_id": 112,
                "prescription_id_id": 7
            }
          ]
}
....
]

Answer 1

You need a filtered Prefetch if you want to filter the related objects only, not the main objects:

from django.db.models import Prefetch

CapturedPrescriptionModel.objects.prefetch_related(Prefetch(
    'antibiotics',
    queryset=SuggestedAntibioticsModel.objects.filter(flag=1)
)

You then have to make sure that antibiotics on the individual prescription objects is only accessed with prescription.antibiotics.all(), otherwise the prefetch is not used and you'll get all antibiotics again.

Answer 2

Collect all Prescriptions:

prescriptions = CapturedPrescriptionModel.objects.all()

for prescription in prescriptions:
    prescription.antibiotics = prescription.antibiotics.filter(flag=1)

# at this time presciptions should be prepared, just make sure to not save them...

You could also extend your model to have a property for that list.

class CapturedPrescriptionModel(ColModel):
    p_id = models.IntegerField()
    p_age = models.IntegerField()
    p_gender = models.CharField(max_length=10)
    p_care_type = models.CharField(max_length=100)
    bacteria_id = models.ForeignKey(BacteriaListModel, 
              on_delete=models.CASCADE, null=True)

    @property
    def flagged_antibiotics(self):
        try:
            return self.antibiotics.filter(flag=1)
        except Exception:
            return []

class SuggestedAntibioticsModel(ColModel):
    prescription_id = models.ForeignKey(CapturedPrescriptionModel, 
                  related_name='antibiotics', 
                   on_delete=models.CASCADE)
    cat_ids = models.TextField()
    flag = models.IntegerField(default=0)

Something like this would be my first take on that