Django access related property dynamically?

Question

I am using getattr to access properties of a model dynamically like so (Assuming the Student model has a property called name):

students = Student.objects.all()
property = 'name'


for student in students:
    print getattr(student, property)

This works fine, however I'm wondering if it is possible to access a property of a related record in the same way, for example (Assuming each Student has a related Group with a property called title):

students = Student.objects.selected_related()
property = 'group.title'


for student in students:
    print getattr(student, property)

With this, I just get the error 'Student has no attribute group.title'

Is there anyway to achieve this?

Any advice appreciated.

Thanks

Don't forget to check if the object in question has the property you're looking for with `hasattr` or handle the potential exception when you try to access an attribute that doesn't exist. — apiguy, Jan 05 '12 at 23:44

score 6 · Accepted Answer · answered Jan 06 '12 at 02:22

While the following code will do what you asked:

students = Student.objects.all()
attr_chain = "group.title".split(".")

for student in students:
    item = student
    for attr in attr_chain:
        item = getattr(item, attr)

    print "%s is in the group %s" % (student, item)

Depending on your needs I would suggest that you look into Django's values_list function on the Queryset class, it can shorten and simplify code in many cases.

name_attr = "name"

#If you look in the documentation you will see why I use "__" here
group_title_attr = "group__title" 

for student_name, group_title in Student.objects.all().values_list(name_attr, group_title_attr):
    print "%s is in the group %s" % (student_name, group_title)

The relevant docs are here and here.

Thanks. All good answers. This one was particularly useful for what I was working on though. — Dan, Jan 06 '12 at 22:04

score 3 · Answer 2 · answered Jan 05 '12 at 23:39

3

looks like you are looking for

getattr(getattr(student, property), subproperty)

you may be able to do with by looping over property.split('.')

answered Jan 05 '12 at 23:39

second

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score 2 · Answer 3 · answered Jul 15 '15 at 07:50

2

you can always use from functools import reduce

and procced like this:

reduce(getattr, "some.nested.property".split("."), student)

answered Jul 15 '15 at 07:50

andilabs

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score 0 · Answer 4 · answered Jan 05 '12 at 23:40

Unless every object in select_related has a property called 'title', that's going to blow up. What does select_related bring back for a Student model? I'm guessing it's more than just a Group object. You'd need to wrap this in a try/ except block or test the object to see if it is of the same type as Group (e.g., isinstance(x, Group)).

What are you actually trying to achieve? This seems a little tortured. Also, I would suggest a re-labelling to make things clearer:

for obj in student_related:
    # print type(obj)
    print getattr(obj, property)

You're not actually getting Student objects in that list.

Can someone explain the downvote? Not sure what's wrong here. — Tom, Jan 06 '12 at 15:36

Django access related property dynamically?

4 Answers4