加入外键表 Django QuerySet

Question

I'm learning Django Queryset, but i get confused when I want to use queryset Django to join table, instead of using Django raw query I want to learn Django queryset.

In my case I want to fetch RiskIdentification data.

class Activity(models.Model):
    act_name           =  models.CharField(max_length = 40)
    created_at          =   models.DateTimeField(auto_now_add=True)
    updated_at          =   models.DateTimeField(auto_now = True)
  

class ActivityDetail(models.Model):
    act_risk_assessment = models.ForeignKey(RiskAssessment, on_delete=models.CASCADE)
    act          = models.ForeignKey(Activity, on_delete=models.CASCADE)
    created_at          = models.DateTimeField(auto_now_add=True)
    updated_at          = models.DateTimeField(auto_now = True)


class RiskAssessment(models.Model):
    name                = models.CharField(max_length = 30)
    date                = models.DateField()
    email_date          = models.DateField()
    created_at          = models.DateTimeField(auto_now_add=True)
    updated_at          = models.DateTimeField(auto_now = True)

class  RiskIdentification (models.Model):
  
    idn_description          = models.CharField(max_length = 30)
    idn_risk_assessment     = models.ForeignKey(RiskAssessment, on_delete=models.CASCADE)
    risk_type             = models.ForeignKey(RiskType, on_delete=models.CASCADE)

class RiskMitigation (models.Model):
  
    mtg_description         = models.CharField(max_length = 30)
    identification_risk     = models.ForeignKey(RiskIdentification, on_delete=models.CASCADE)

It will look like this in raw query

SELECT  idn_description
FROM RiskAsessment RA 
JOIN RiskIdentification RI on RA.id = RI.idn_riskAsessment 
JOIN ActivityDetail AD on AD.act_risk_assessment = RA.id 
JOIN Activity A on A.id = AD.act

Please help me how Django query will looks like, and describe it.

Answer 1

A queryset equivalent to your SQL is:

qs = (RiskIdentification.objects
      .exclude(idn_risk_assessment__activity_detail_set=None)
      .values('idn_description'))

It is usually not necassary to restrict output fields by .values(...) .

It is always useful in a question to express the expected result also in human language although it could be not as accurate as SQL or queryset, but there is less risk of mistake or misconception.

An advantage of writing querysets instead of raw SQL is that the correctness of queryset can be verified by compiling to SQL and reading. Many mistakes can be found by this, while the correctness of raw SQL can't be checked no independent way:

>>> print(str(queryset.query))
SELECT ... FROM ...

I'm trying to guess your intention from your SQL:

I want to know all idn_description from RiskIdentification that are used in any ActivityDetail . I want to see them so many time how much they are used in any ActivityDetail .

(The part of SQL JOIN Activity A on A.id = AD.act is useless if no Activity field is used for output or for WHERE conditions and the database has been created correctly with integrity rules (automatically by Django) then a missing or invalid value is always rejected by database because that foreign key is required.)

It is useful to define a related_name for act_risk_assessment field, eg "activity_details", or expect for now that it is the default name activity_detail_set for now.