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Version 3 (modified by public@…, 7 years ago) (diff)

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Schema Evolution Documentation

Introduction

Schema evolution is the function of updating an existing Django generated database schema to a newer/modified version based upon a newer/modified set of Django models.

Limitations

I feel it important to note that is an automated implementation designed to handle schema evolution, not revolution. No tool, other than storing DBA written SQL scripts and auto-applying them via schema versioning or DB fingerprinting (which is a trivial solution - I have a Java implementation if anyone wants it), can handle the full scope of possible database changes. Once you accept this fact, the following becomes self-evident:

  • There is a trade off between ease of use and the scope of coverable problems.

Combine that with:

  • The vast majority of database changes are minor, evolutionary tweaks. (*)
  • Very few people are DBAs.

And I believe the ideal solution is in easing the life of common Django developer, not in appeasing the DBA's or power-developer's desire for an all-in-one-comprehensive solution. Massive schema changes (w/ data retention) are always going to require someone with database skill, but we can empower the people to do the simple things for themselves.

(*) By this I mean adding/removing/renaming tables and adding/removing/renaming/changing-attributes-of columns.

Downloading / Installing

This functionality is not yet in Django/trunk, but in a separate schema-evolution branch. To download this branch, run the following:

svn co http://code.djangoproject.com/svn/django/schema-evolution/ django_se_src
ln -s `pwd`/django_se_src/django SITE-PACKAGES-DIR/django

Or, if you're currently running Django v0.96, run the following:

cd /<path_to_python_dir>/site-packages/django/
wget http://kered.org/blog/wp-content/uploads/2007/07/django_schema_evolution-v096patch.txt
patch -p1 < django_schema_evolution-v096patch.txt

The last command will produce the following output:

patching file core/management.py
patching file db/backends/mysql/base.py
patching file db/backends/mysql/introspection.py
patching file db/backends/postgresql/base.py
patching file db/backends/postgresql/introspection.py
patching file db/backends/sqlite3/base.py
patching file db/backends/sqlite3/introspection.py
patching file db/models/fields/__init__.py
patching file db/models/options.py

How To Use

For the most part, schema evolution is designed to be automagic via introspection. Make changes to your models, run syncdb, and you're done. But like all schema changes, it's wise to preview what is going to be run. To do this, run the following:

./manage sqlevolve app_name

This will output to the command line the SQL to be run to bring your database schema up to date with your model structure.

However not everything can be handled through introspection. A small amount of metadata is used in the cases of model or field renames, so that the introspection code can match up the old field to the new field. (therefore preserving your data)

For renaming a column, use an "aka" attribute:

    # this field used to be called pub_date
    publish_date = models.DateTimeField('date published', aka='pub_date')

If you have renamed this twice and still wish to support migration from both older schemas, "aka"s can be tuples:

    # this field used to be called pub_date
    publish_date = models.DateTimeField('date published', aka=('pub_date','other_old_field_name'))

For renaming a model, add an "aka" field to the Meta section:

# the original name for this model was 'Choice'
class Option(models.Model):
    [...]
    class Meta:
        aka = 'Choice'

For further examples...

Usage Examples

The following documentation will take you through several common model changes and show you how Django's schema evolution handles them. Each example provides the pre and post model source code, as well as the SQL output.

Adding / Removing Fields

Model: version 1

    from django.db import models
    
    class Poll(models.Model):
        question = models.CharField(maxlength=200)
        pub_date = models.DateTimeField('date published')
        author = models.CharField(maxlength=200)
        def __str__(self):
            return self.question
        
    class Choice(models.Model):
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        votes = models.IntegerField()
        def __str__(self):
            return self.choice

Model: version 2

    from django.db import models
    
    class Poll(models.Model):
        question = models.CharField(maxlength=200)
        pub_date = models.DateTimeField('date published')
        author = models.CharField(maxlength=200)
        def __str__(self):
            return self.question
    
        # new fields
        pub_date2 = models.DateTimeField('date published')

    class Choice(models.Model):
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        votes = models.IntegerField()
        def __str__(self):
            return self.choice
    
        # new fields
        votes2 = models.IntegerField()
        hasSomething = models.BooleanField()
        creatorIp = models.IPAddressField()

Output: v1⇒v2

    BEGIN;
    ALTER TABLE `case01_add_field_poll` ADD COLUMN `pub_date2` datetime NOT NULL;
    ALTER TABLE `case01_add_field_choice` ADD COLUMN `votes2` integer NOT NULL;
    ALTER TABLE `case01_add_field_choice` ADD COLUMN `hasSomething` bool NOT NULL;
    ALTER TABLE `case01_add_field_choice` ADD COLUMN `creatorIp` char(15) NOT NULL;
    COMMIT;

Output: v2⇒v1

    -- warning: as the following may cause data loss, it/they must be run manually
    -- ALTER TABLE `case01_add_field_poll` DROP COLUMN `pub_date2`;
    -- end warning
    -- warning: as the following may cause data loss, it/they must be run manually
    -- ALTER TABLE `case01_add_field_choice` DROP COLUMN `votes2`;
    -- ALTER TABLE `case01_add_field_choice` DROP COLUMN `creatorIp`;
    -- ALTER TABLE `case01_add_field_choice` DROP COLUMN `hasSomething`;
    -- end warning

Renaming Fields

Model: version 1

    from django.db import models

    class Poll(models.Model):
        """this model originally had fields named pub_date and the_author.  you can use 
        either a str or a tuple for the aka value.  (tuples are used if you have changed 
        its name more than once)"""
        question = models.CharField(maxlength=200)
        pub_date = models.DateTimeField('date published', aka='publish_date')
        the_author = models.CharField(maxlength=200, aka='the_author')
        def __str__(self):
            return self.question
        
    class Choice(models.Model):
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        votes = models.IntegerField(aka='votes')
        def __str__(self):
            return self.choice

Model: version 2

    from django.db import models
    
    class Poll(models.Model):
        """this model originally had fields named pub_date and the_author.  you can use
        either a str or a tuple for the aka value.  (tuples are used if you have changed
        its name more than once)"""
        question = models.CharField(maxlength=200)
        published_date = models.DateTimeField('date published', aka=('pub_date', 'publish_date'))
        author = models.CharField(maxlength=200, aka='the_author')
        def __str__(self):
            return self.question
        
    class Choice(models.Model):
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        number_of_votes = models.IntegerField(aka='votes')
        def __str__(self):
            return self.choice

Output: v1⇒v2

    BEGIN;
    ALTER TABLE `case02_rename_field_poll` CHANGE COLUMN `pub_date` `published_date` datetime NOT NULL;
    ALTER TABLE `case02_rename_field_poll` CHANGE COLUMN `the_author` `author` varchar(200) NOT NULL;
    ALTER TABLE `case02_rename_field_choice` CHANGE COLUMN `votes` `number_of_votes` integer NOT NULL;
    COMMIT;

Renaming Models

Model: version 1

    from django.db import models
    
    class Poll(models.Model):
        question = models.CharField(maxlength=200)
        pub_date = models.DateTimeField('date published')
        author = models.CharField(maxlength=200)
        def __str__(self):
            return self.question
        
    class Choice(models.Model):
        "the original name for this model was 'Choice'"
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        number_of_votes = models.IntegerField()
        def __str__(self):
            return self.choice
        class Meta:
            aka = ('Choice', 'OtherBadName')

Model: version 2

    from django.db import models
    
    class Poll(models.Model):
        question = models.CharField(maxlength=200)
        pub_date = models.DateTimeField('date published')
        author = models.CharField(maxlength=200)
        def __str__(self):
            return self.question
        
    class Option(models.Model):
        "the original name for this model was 'Choice'"
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        # show that field name changes work too
        votes = models.IntegerField(aka='number_of_votes')
        def __str__(self):
            return self.choice
        class Meta:
            aka = ('Choice', 'BadName')

Output: v1⇒v2

    BEGIN;
    ALTER TABLE `case03_rename_model_choice` RENAME TO `case03_rename_model_option`;
    ALTER TABLE `case03_rename_model_option` CHANGE COLUMN `number_of_votes` `votes` integer NOT NULL;
    COMMIT;

Changing Flags

Model: version 1

    from django.db import models
    
    class Poll(models.Model):
        question = models.CharField(maxlength=200)
        pub_date = models.DateTimeField('date published')
        author = models.CharField(maxlength=200)
        def __str__(self):
            return self.question
        
    class Choice(models.Model):
        "the original name for this model was 'Choice'"
        poll = models.ForeignKey(Poll)
        choice = models.CharField(maxlength=200)
        votes = models.IntegerField()
        def __str__(self):
            return self.choice

    class Foo(models.Model):
        GENDER_CHOICES = (
            ('M', 'Male'),
            ('F', 'Female'),
        )
        gender = models.CharField(maxlength=1, choices=GENDER_CHOICES)

Model: version 2

    from django.db import models
    
    class Poll(models.Model):
        question = models.CharField(maxlength=100)
        pub_date = models.DateTimeField('date published')
        author = models.CharField(maxlength=200)
        def __str__(self):
            return self.question
        
    class Choice(models.Model):
        "the original name for this model was 'Choice'"
        poll = models.ForeignKey(Poll)
        # make sure aka still works with a flag change
        option = models.CharField(maxlength=400, aka='choice')
        votes = models.IntegerField()
        votes2 = models.IntegerField() # make sure column adds still work
        def __str__(self):
            return self.choice
    
    class Foo(models.Model):
        GENDER_CHOICES = (
            ('M', 'Male'),
            ('F', 'Female'),
        )
        gender = models.CharField(maxlength=1, choices=GENDER_CHOICES, db_index=True)
        gender2 = models.CharField(maxlength=1, null=True, unique=True)
            

Output: v1⇒v2

    BEGIN;
    ALTER TABLE `case04_change_flag_poll` MODIFY COLUMN `question` varchar(100) NOT NULL;
    ALTER TABLE `case04_change_flag_foo` ADD COLUMN `gender2` varchar(1) NULL UNIQUE;
    ALTER TABLE `case04_change_flag_choice` MODIFY COLUMN `choice` varchar(400) NOT NULL;
    ALTER TABLE `case04_change_flag_choice` CHANGE COLUMN `choice` `option` varchar(400) NOT NULL;
    ALTER TABLE `case04_change_flag_choice` ADD COLUMN `votes2` integer NOT NULL;
    COMMIT;

Criticisms

I _really_ don't like the aka representations in the model. The models file should always be a clean statement of the current state of the model. Migration is about getting an old database to match the currently required models - if I don't have a legacy database, I don't really want the cruft hanging around in my models. Migration plans or historical models really should be kept out of the core model file, IMHO.

We currently store all sorts of non-DB related metadata in the model that arguably should not be there, including presentation information. We do this for clarity and convenience - you would have to duplicate a lot of information otherwise in multiple locations without any obvious direct connection. So which paradigm is more critical here, DRY or MVC? Or do we continue with the status-quo of a common-sense balance? As far as cruft, if you don't have a legacy database, you wouldn't have any aka fields to begin with. And as you phase out legacy support, simply delete them.

Unless I'm missing something, the aka approach doesn't track exactly which name versions correlate with other versions. Consider; your aka chain for one field could indicate a rename from 'foo' to 'bar' to 'foo' to 'bar'; a second field could indicate renames from 'whiz' to 'foo' to 'whiz', etc. A tuple of historical names doesn't tell me which sets of names were in use concurrently; so if I find a database with a field called 'foo' which requires migration, is 'foo' the first field or the second?

Correct, however I thought this to be a highly unlikely scenario, not warranting the extra notational complexity. But just as we support strings and tuples, there is nothing to say we can't extend it to support say a mapping of historical names to date ranges, if the need arises.

Future Work

The biggest missing piece I believe to be changing column types. For instance, say you currently have:

    ssn = models.IntegerField()

Which you want to change into:

    ssn = models.CharField(maxlength=12)

Schema evolution should generate SQL to add the new column, push the data from the old to the new column, then delete the old column. Warnings should be provided for completely incompatible types or other loss-of-information scenarios.

Conclusion

That's pretty much it. If you can suggest additional examples or test cases you think would be of value, please email me at public@….