Thanks for the suggestions..
okay suppose i change my models to something like this
class Student_Info(models.Model):
Roll_No = models.IntegerField(primary_key=True)
Cell_No = models.IntegerField()
E_mail = models.EmailField(max_length=30)
class Academics(models.Model):
#Roll_No = models.ForeignKey('Student_Info',to_field='Roll_No')
roll = models.OneToOneField('Student_Info',to_field='Roll_No')
GPA = models.IntegerField()
Now how can i access the academic fields through the student object
>>> from DbDemo.models import Student_Info
>>> list=Student_Info.objects.all()
>>> obj=list[0]
>>> obj
<Student_Info: Student_Info object>
Now i got the Student_info object. and i know Student_Info is linked to
Academics via Roll_No..
so now how could i read the GPA in Academics any suggestions??
Date: Tue, 9 Aug 2011 08:50:48 -0400
Subject: Re: Need help writing join Django query for this simple model..
From: kmtra...@gmail.com
To: django-users@googlegroups.com
On Mon, Aug 8, 2011 at 11:49 PM, Hayyan Rafiq <hayya...@hotmail.com> wrote:
so i tried this now
class Student_Info(models.Model):
Roll_No = models.IntegerField(primary_key=True)
Cell_No = models.IntegerField()
E_mail = models.EmailField(max_length=30)
class Academics(models.Model):
#Roll_No = models.ForeignKey('Student_Info',to_field='Roll_No')
Roll_No = models.OneToOneField('Student_Info',to_field='Roll_No')
GPA = models.IntegerField()
You would really be better off naming Roll_No in Academics something like
"student_info". (And in general changing your naming conventions to match
Python, but this one field in particular is going to be very confusing if you
name it as you have). In the Django ORM, the field will be an object that
contains the associated Student_Info instance, it will not be a simple Integer
containing the Roll_No of that student. The fact that you have specified
Roll_No as the field to link on (which is unnecessary by the way, the primary
key field will be used by default) is irrelevant.
Then i tried this :
>>> from DbDemo.models import Student_Info
>>> list=Student_Info.objects.all()
>>> obj=list[0]
>>> obj
<Student_Info: Student_Info object>
>>> obj.GPA
>>> obj.GPA
Traceback (most recent call last):
File "<console>", line 1, in <module>
AttributeError: 'Student_Info' object has no attribute 'GPA'
GPA isn't an attribute of a Student_Info, it's an Attribute of Academics.
Student_Info has an associated academics object due to the OneToOne field in
Academics, to access it's fields you need to specify that is where the field is:
obj.academics.GPA
The naming of the field used to access the "backwards" relationship is noted
here:
https://docs.djangoproject.com/en/dev/topics/db/queries/#one-to-one-relationships
Then i tried
>>> from DbDemo.models import Academics
>>> list= Academics.objects.all()
>>> obj=list[0]
>>> obj
<Academics: Academics object>
>>> obj.E_mail
Traceback (most recent call last):
File "<console>", line 1, in <module>
AttributeError: 'Academics' object has no attribute 'E_mail'
E_mail is in the linked Student_Info object:
obj.Roll_No.E_mail
(this is why I mentioned Roll_No is probably not really what you want to name
that field)
>>> obj.Student_Info.GPA
Traceback (most recent call last):
File "<console>", line 1, in <module>
AttributeError: 'Academics' object has no attribute 'Student_Info'
GPA is directly in the Academics object:
obj.GPA
Linking two models via a ForeignKey or OneToOne field doesn't make all the
attributes of one directly accessible in the other: you need to specify the
relationship you want to follow in order to get at whatever attribute you are
looking for.
Karen
--
http://tracey.org/kmt/
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