Comments on: Schema Evolution Confusion / Example Case http://kered.org/blog/2007-08-03/schema-evolution-confusion-example-case/ Just another WordPress weblog Fri, 21 Nov 2008 20:11:21 +0000 http://wordpress.org/?v=2.0.5 by: buriy http://kered.org/blog/2007-08-03/schema-evolution-confusion-example-case/#comment-11989 Mon, 06 Aug 2007 03:14:08 +0000 http://kered.org/blog/2007-08-03/schema-evolution-confusion-example-case/#comment-11989 IMO the right behavior is to ask user to confirm data loss when changing v1 => v2. IMO the right behavior is to ask user to confirm data loss when changing v1 => v2.

]]> by: SmileyChris http://kered.org/blog/2007-08-03/schema-evolution-confusion-example-case/#comment-11897 Fri, 03 Aug 2007 21:48:35 +0000 http://kered.org/blog/2007-08-03/schema-evolution-confusion-example-case/#comment-11897 I would argue that the exact path from v1 => v3 can definitely be important! v1 class Russ(models.Model): ____"this model is going to have a bit of a day (v1)" ____a = models.CharField(maxlength=200) v2 class Russ(models.Model): ____"this model is going to have a bit of a day (v2)" ____b = models.CharField(maxlength=200, null=True) ____c = models.CharField(maxlength=200, aka='a') v3 class Russ(models.Model): ____"this model is going to have a bit of a day (v3)" ____a = models.CharField(maxlength=200, aka='b', null=True) ____b = models.CharField(maxlength=200, aka='c') Tester 1 & 2 have the same database contents at v1: [Russ(a='first')] Tester 1 goes from v1 => v2 => v3. Now he has [Russ(a=None, b='first')] Tester 2 merges from 1's work and evolves from v1 => v3. Now he has [Russ(a='first', b=None)] (or error since b shouldn't be able to be NULL). Since you have "lost" what happened in v2, it's not possible for Tester 2 to guess what should have happened. I would argue that the exact path from v1 => v3 can definitely be important!

v1

class Russ(models.Model):
____”this model is going to have a bit of a day (v1)”
____a = models.CharField(maxlength=200)

v2

class Russ(models.Model):
____”this model is going to have a bit of a day (v2)”
____b = models.CharField(maxlength=200, null=True)
____c = models.CharField(maxlength=200, aka=’a')

v3

class Russ(models.Model):
____”this model is going to have a bit of a day (v3)”
____a = models.CharField(maxlength=200, aka=’b', null=True)
____b = models.CharField(maxlength=200, aka=’c')

Tester 1 & 2 have the same database contents at v1: [Russ(a=’first’)]

Tester 1 goes from v1 => v2 => v3. Now he has [Russ(a=None, b=’first’)]
Tester 2 merges from 1’s work and evolves from v1 => v3. Now he has [Russ(a=’first’, b=None)] (or error since b shouldn’t be able to be NULL).

Since you have “lost” what happened in v2, it’s not possible for Tester 2 to guess what should have happened.

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