Join request in django between three tables and display all attributes
I have three models
class A(models.Model):
field1 = models.IntegerField()
class B(models.Model):
id_a = models.ForeignKey(A,on_delete=models.CASCADE)
field1 = models.IntegerField()
field2 = models.IntegerField()
class C(models.Model):
id_a = models.ForeignKey(A,on_delete=models.CASCADE)
field1 = models.IntegerField()
field2 = models.IntegerField()
I want to write a request that looks like this: SELECT * FROM B,C,A WHERE B.id_a=C.id_a WHERE A.id_a=2 and display all the attributes of the two tables
Here is what I tried to do:
a_id_att = 1
data = B.objects.filter(id_a=C.objects.filter(id_a=a_id_att)[0])
It does not work. How to write the join and make to display all the attributes of the tables?
The SQL statement that you wrote seems strange.
SELECT * FROM B, C, A
WHERE B.id_a = C.id_a
AND A.id_a = 2
It seems that you want a single row from A
and then all related rows from B
and C
, which your SQL query does NOT achieve.
Did you mean something like this:
SELECT * FROM B, C, A
WHERE A.id = 2
AND B.id_a = A.id
AND C.id_a = A.id
You can achieve something like that in Django using prefetch_related()
, which builds a query so that the related rows are also loaded into memory in the first query and not in subsequent queries.
# this will return a queryset with a single element, or empty
qs = A.objects.prefetch_related('b_set', 'c_set').filter(id=2)
for elem in qs: # here the single DB query is made
print(elem.field1) # A.field1
for det in elem.b_set.all():
print(det.field1) # B.field1, does NOT make another DB query
print(det.field2) # B.field2, does NOT make another DB query
for det in elem.c_set.all():
print(det.field1) # C.field1, does NOT make another DB query
print(det.field2) # C.field2, does NOT make another DB query
Note: I use b_set
here because that is the default for the ForeignKey
field; this changes if the field would specify a different related_name
.
Does this address and solve your issue?