Comparing 2 model entities

Question

Is there an easy way to compare to entities to to check for property value differences? I want something like this:

me = User.get_by_id( 28 )
cloned_me = me

cloned_me.first_name = 'Tom'

if me != cloned_me:
    self.response.out.write( 'These 2 objects have different property values' )

`cloned_me` isn't a copy here - it's simply another reference to the same object. This check will always return `True`. — Nick Johnson, Sep 09 '11 at 00:50
possible duplicate of [Elegant way to avoid .put() on unchanged entities](http://stackoverflow.com/questions/7330974/elegant-way-to-avoid-put-on-unchanged-entities) — Nick Johnson, Sep 09 '11 at 00:52

Anurag Uniyal · Accepted Answer · 2011-09-08T16:03:25.953

0

For simplest scenario you can compare objects field by field e.g.

from django.contrib.auth.models import User

def compare(user1, user2):
    for field in user1._meta.fields: 
        val1 = getattr(user1, field.name)
        val2 = getattr(user2, field.name)
        if val1 != val2:
            print "%s differ '%s' != '%s'"%(field.name, val1, val2)
            break

compare(User(username="anurag"), User(username="uniyal"))

output:

username differ 'anurag' != 'uniyal'

You can later on improve it if you need to further compare foerign keys etc

and i think you are aware that in your example clone_me is not actually a clone of me, it is me

edited Sep 08 '11 at 16:03

answered Sep 08 '11 at 15:55

Anurag Uniyal

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So to make a clone I would use copy.copy? – zakdances Sep 08 '11 at 17:05
for cloning see http://stackoverflow.com/questions/4733609/how-do-i-clone-a-django-model-instance-object-and-save-it-to-the-database – Anurag Uniyal Sep 08 '11 at 17:55

score 0 · Answer 2 · answered Sep 08 '11 at 16:15

0

Try using sets:

differences = set(me.__dict__) ^ set(cloned_me.__dict__)
if differences:
    self.response.out.write('These 2 objects have different property values')

You could even output the attributes that were different (they're contained in the new differences set).

answered Sep 08 '11 at 16:15

Jordan Bouvier

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Is this an efficient method of comparing? From my understanding, bitwise comparisons are expensive. – zakdances Sep 08 '11 at 17:06
I'm not sure how expensive it is. The time difference between this and anything possibly faster is probably negligible unless you have a very large data set. – Jordan Bouvier Sep 08 '11 at 19:12

Comparing 2 model entities

2 Answers2