Python 实用宝典

Question 1

So about a year ago I started a project and like all new developers I didn’t really focus too much on the structure, however now I am further along with Django it has started to appear that my project layout mainly my models are horrible in structure.

I have models mainly held in a single app and really most of these models should be in their own individual apps, I did try and resolve this and move them with south however I found it tricky and really difficult due to foreign keys ect.

However due to Django 1.7 and built in support for migrations is there a better way to do this now?

Question 2

I am removing the old answer as may result in data loss. As ozan mentioned, we can create 2 migrations one in each app. The comments below this post refer to my old answer.

First migration to remove model from 1st app.

$ python manage.py makemigrations old_app --empty

Edit migration file to include these operations.

class Migration(migrations.Migration):

    database_operations = [migrations.AlterModelTable('TheModel', 'newapp_themodel')]

    state_operations = [migrations.DeleteModel('TheModel')]

    operations = [
      migrations.SeparateDatabaseAndState(
        database_operations=database_operations,
        state_operations=state_operations)
    ]

Second migration which depends on first migration and create the new table in 2nd app. After moving model code to 2nd app

$ python manage.py makemigrations new_app

and edit migration file to something like this.

class Migration(migrations.Migration):

    dependencies = [
        ('old_app', 'above_migration')
    ]

    state_operations = [
        migrations.CreateModel(
            name='TheModel',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
            ],
            options={
                'db_table': 'newapp_themodel',
            },
            bases=(models.Model,),
        )
    ]

    operations = [
        migrations.SeparateDatabaseAndState(state_operations=state_operations)
    ]

Question 3

This can be done fairly easily using migrations.SeparateDatabaseAndState. Basically, we use a database operation to rename the table concurrently with two state operations to remove the model from one app’s history and create it in another’s.

Remove from old app

python manage.py makemigrations old_app --empty

In the migration:

class Migration(migrations.Migration):

    dependencies = []

    database_operations = [
        migrations.AlterModelTable('TheModel', 'newapp_themodel')
    ]

    state_operations = [
        migrations.DeleteModel('TheModel')
    ]

    operations = [
        migrations.SeparateDatabaseAndState(
            database_operations=database_operations,
            state_operations=state_operations)
    ]

Add to new app

First, copy the model to the new app’s model.py, then:

python manage.py makemigrations new_app

This will generate a migration with a naive CreateModel operation as the sole operation. Wrap that in a SeparateDatabaseAndState operation such that we don’t try to recreate the table. Also include the prior migration as a dependency:

class Migration(migrations.Migration):

    dependencies = [
        ('old_app', 'above_migration')
    ]

    state_operations = [
        migrations.CreateModel(
            name='TheModel',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
            ],
            options={
                'db_table': 'newapp_themodel',
            },
            bases=(models.Model,),
        )
    ]

    operations = [
        migrations.SeparateDatabaseAndState(state_operations=state_operations)
    ]

Question 4

I encountered the same problem. Ozan’s answer helped me a lot but unfortunately was not enough. Indeed I had several ForeignKey linking to the model I wanted to move. After some headache I found the solution so decided to post it to solve people time.

You need 2 more steps:

Before doing anything, change all your ForeignKey linking to TheModel into Integerfield. Then run python manage.py makemigrations
After doing Ozan’s steps, re-convert your foreign keys: put back ForeignKey(TheModel)instead of IntegerField(). Then make the migrations again (python manage.py makemigrations). You can then migrate and it should work (python manage.py migrate)

Hope it helps. Of course test it in local before trying in production to avoid bad suprises :)

Question 5

How I did it (tested on Django==1.8, with postgres, so probably also 1.7)

Situation

app1.YourModel

but you want it to go to: app2.YourModel

Copy YourModel (the code) from app1 to app2.

add this to app2.YourModel:

Class Meta:
    db_table = 'app1_yourmodel'

$ python manage.py makemigrations app2
A new migration (e.g. 0009_auto_something.py) is made in app2 with a migrations.CreateModel() statement, move this statement to the initial migration of app2 (e.g. 0001_initial.py) (it will be just like it always have been there). And now remove the created migration = 0009_auto_something.py
Just as you act, like app2.YourModel always has been there, now remove the existence of app1.YourModel from your migrations. Meaning: comment out the CreateModel statements, and every adjustment or datamigration you used after that.
And of course, every reference to app1.YourModel has to be changed to app2.YourModel through your project. Also, don’t forget that all possible foreign keys to app1.YourModel in migrations have to be changed to app2.YourModel
Now if you do $ python manage.py migrate, nothing has changed, also when you do $ python manage.py makemigrations, nothing new has been detected.
Now the finishing touch: remove the Class Meta from app2.YourModel and do $ python manage.py makemigrations app2 && python manage.py migrate app2 (if you look into this migration you’ll see something like this:)
```
    migrations.AlterModelTable(
    name='yourmodel',
    table=None,
),
```

table=None, means it will take the default table-name, which in this case will be app2_yourmodel.

DONE, with data saved.

P.S during the migration it will see that that content_type app1.yourmodel has been removed and can be deleted. You can say yes to that but only if you don’t use it. In case you heavily depend on it to have FKs to that content-type be intact, don’t answer yes or no yet, but go into the db that time manually, and remove the contentype app2.yourmodel, and rename the contenttype app1.yourmodel to app2.yourmodel, and then continue by answering no.

Question 6

I get nervous hand-coding migrations (as is required by Ozan’s answer) so the following combines Ozan’s and Michael’s strategies to minimize the amount of hand-coding required:

Before moving any models, make sure you’re working with a clean baseline by running makemigrations.
Move the code for the Model from app1 to app2
As recommended by @Michael, we point the new model to the old database table using the db_table Meta option on the “new” model:
```
class Meta:
    db_table = 'app1_yourmodel'
```
Run makemigrations. This will generate CreateModel in app2 and DeleteModel in app1. Technically, these migrations refer to the exact same table and would remove (including all data) and re-create the table.
In reality, we don’t want (or need) to do anything to the table. We just need Django to believe that the change has been made. Per @Ozan’s answer, the state_operations flag in SeparateDatabaseAndState does this. So we wrap all of the migrations entries IN BOTH MIGRATIONS FILES with SeparateDatabaseAndState(state_operations=[...]). For example,
```
operations = [
    ...
    migrations.DeleteModel(
        name='YourModel',
    ),
    ...
]
```
becomes
```
operations = [
    migrations.SeparateDatabaseAndState(state_operations=[
        ...
        migrations.DeleteModel(
            name='YourModel',
        ),
        ...
    ])
]
```
You also need to make sure the new “virtual” CreateModel migration depends on any migration that actually created or altered the original table. For example, if your new migrations are app2.migrations.0004_auto_<date> (for the Create) and app1.migrations.0007_auto_<date> (for the Delete), the simplest thing to do is:
- Open app1.migrations.0007_auto_<date> and copy its app1 dependency (e.g. ('app1', '0006...'),). This is the “immediately prior” migration in app1 and should include dependencies on all of the actual model building logic.
- Open app2.migrations.0004_auto_<date> and add the dependency you just copied to its dependencies list.

If you have ForeignKey relationship(s) to the model you’re moving, the above may not work. This happens because:

Dependencies are not automatically created for the ForeignKey changes
We do not want to wrap the ForeignKey changes in state_operations so we need to ensure they are separate from the table operations.

NOTE: Django 2.2 added a warning (models.E028) that breaks this method. You may be able to work around it with managed=False but I have not tested it.

The “minimum” set of operations differ depending on the situation, but the following procedure should work for most/all ForeignKey migrations:

COPY the model from app1 to app2, set db_table, but DON’T change any FK references.
Run makemigrations and wrap all app2 migration in state_operations (see above)
- As above, add a dependency in the app2 CreateTable to the latest app1 migration
Point all of the FK references to the new model. If you aren’t using string references, move the old model to the bottom of models.py (DON’T remove it) so it doesn’t compete with the imported class.
Run makemigrations but DON’T wrap anything in state_operations (the FK changes should actually happen). Add a dependency in all the ForeignKey migrations (i.e. AlterField) to the CreateTable migration in app2 (you’ll need this list for the next step so keep track of them). For example:
- Find the migration that includes the CreateModel e.g. app2.migrations.0002_auto_<date> and copy the name of that migration.
- Find all migrations that have a ForeignKey to that model (e.g. by searching app2.YourModel to find migrations like:
```
class Migration(migrations.Migration):

    dependencies = [
        ('otherapp', '0001_initial'),
    ]

    operations = [
        migrations.AlterField(
            model_name='relatedmodel',
            name='fieldname',
            field=models.ForeignKey(... to='app2.YourModel'),
        ),
    ]
```
- Add the CreateModel migration as as a dependency:
```
class Migration(migrations.Migration):

    dependencies = [
        ('otherapp', '0001_initial'),
        ('app2', '0002_auto_<date>'),
    ]  
```
Remove the models from app1
Run makemigrations and wrap the app1 migration in state_operations.
- Add a dependency to all of the ForeignKey migrations (i.e. AlterField) from the previous step (may include migrations in app1 and app2).
- When I built these migrations, the DeleteTable already depended on the AlterField migrations so I didn’t need to manually enforce it (i.e. Alter before Delete).

At this point, Django is good to go. The new model points to the old table and Django’s migrations have convinced it that everything has been relocated appropriately. The big caveat (from @Michael’s answer) is that a new ContentType is created for the new model. If you link (e.g. by ForeignKey) to content types, you’ll need to create a migration to update the ContentType table.

I wanted to cleanup after myself (Meta options and table names) so I used the following procedure (from @Michael):

Remove the db_table Meta entry
Run makemigrations again to generate the database rename
Edit this last migration and make sure it depends on the DeleteTable migration. It doesn’t seem like it should be necessary as the Delete should be purely logical, but I’ve run into errors (e.g. app1_yourmodel doesn’t exist) if I don’t.

Question 7

Another hacky alternative if the data is not big or too complicated, but still important to maintain, is to:

Get data fixtures using manage.py dumpdata
Proceed to model changes and migrations properly, without relating the changes
Global replace the fixtures from the old model and app names to the new
Load data using manage.py loaddata

Question 8

Copied from my answer at https://stackoverflow.com/a/47392970/8971048

In case you need to move the model and you don’t have access to the app anymore (or you don’t want the access), you can create a new Operation and consider to create a new model only if the migrated model does not exist.

In this example I am passing ‘MyModel’ from old_app to myapp.

class MigrateOrCreateTable(migrations.CreateModel):
    def __init__(self, source_table, dst_table, *args, **kwargs):
        super(MigrateOrCreateTable, self).__init__(*args, **kwargs)
        self.source_table = source_table
        self.dst_table = dst_table

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        table_exists = self.source_table in schema_editor.connection.introspection.table_names()
        if table_exists:
            with schema_editor.connection.cursor() as cursor:
                cursor.execute("RENAME TABLE {} TO {};".format(self.source_table, self.dst_table))
        else:
            return super(MigrateOrCreateTable, self).database_forwards(app_label, schema_editor, from_state, to_state)


class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0002_some_migration'),
    ]

    operations = [
        MigrateOrCreateTable(
            source_table='old_app_mymodel',
            dst_table='myapp_mymodel',
            name='MyModel',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('name', models.CharField(max_length=18))
            ],
        ),
    ]

Question 9

This is tested roughly, so do not forget to backup your DB!!!

For example, there are two apps: src_app and dst_app, we want to move model MoveMe from src_app to dst_app.

Create empty migrations for both apps:

python manage.py makemigrations --empty src_app
python manage.py makemigrations --empty dst_app

Let’s assume, that new migrations are XXX1_src_app_new and XXX1_dst_app_new, previuos top migrations are XXX0_src_app_old and XXX0_dst_app_old.

Add an operation that renames table for MoveMe model and renames its app_label in ProjectState to XXX1_dst_app_new. Do not forget to add dependency on XXX0_src_app_old migration. The resulting XXX1_dst_app_new migration is:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations

# this operations is almost the same as RenameModel
# https://github.com/django/django/blob/1.7/django/db/migrations/operations/models.py#L104
class MoveModelFromOtherApp(migrations.operations.base.Operation):

    def __init__(self, name, old_app_label):
        self.name = name
        self.old_app_label = old_app_label

    def state_forwards(self, app_label, state):

        # Get all of the related objects we need to repoint
        apps = state.render(skip_cache=True)
        model = apps.get_model(self.old_app_label, self.name)
        related_objects = model._meta.get_all_related_objects()
        related_m2m_objects = model._meta.get_all_related_many_to_many_objects()
        # Rename the model
        state.models[app_label, self.name.lower()] = state.models.pop(
            (self.old_app_label, self.name.lower())
        )
        state.models[app_label, self.name.lower()].app_label = app_label
        for model_state in state.models.values():
            try:
                i = model_state.bases.index("%s.%s" % (self.old_app_label, self.name.lower()))
                model_state.bases = model_state.bases[:i] + ("%s.%s" % (app_label, self.name.lower()),) + model_state.bases[i+1:]
            except ValueError:
                pass
        # Repoint the FKs and M2Ms pointing to us
        for related_object in (related_objects + related_m2m_objects):
            # Use the new related key for self referential related objects.
            if related_object.model == model:
                related_key = (app_label, self.name.lower())
            else:
                related_key = (
                    related_object.model._meta.app_label,
                    related_object.model._meta.object_name.lower(),
                )
            new_fields = []
            for name, field in state.models[related_key].fields:
                if name == related_object.field.name:
                    field = field.clone()
                    field.rel.to = "%s.%s" % (app_label, self.name)
                new_fields.append((name, field))
            state.models[related_key].fields = new_fields

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        old_apps = from_state.render()
        new_apps = to_state.render()
        old_model = old_apps.get_model(self.old_app_label, self.name)
        new_model = new_apps.get_model(app_label, self.name)
        if self.allowed_to_migrate(schema_editor.connection.alias, new_model):
            # Move the main table
            schema_editor.alter_db_table(
                new_model,
                old_model._meta.db_table,
                new_model._meta.db_table,
            )
            # Alter the fields pointing to us
            related_objects = old_model._meta.get_all_related_objects()
            related_m2m_objects = old_model._meta.get_all_related_many_to_many_objects()
            for related_object in (related_objects + related_m2m_objects):
                if related_object.model == old_model:
                    model = new_model
                    related_key = (app_label, self.name.lower())
                else:
                    model = related_object.model
                    related_key = (
                        related_object.model._meta.app_label,
                        related_object.model._meta.object_name.lower(),
                    )
                to_field = new_apps.get_model(
                    *related_key
                )._meta.get_field_by_name(related_object.field.name)[0]
                schema_editor.alter_field(
                    model,
                    related_object.field,
                    to_field,
                )

    def database_backwards(self, app_label, schema_editor, from_state, to_state):
        self.old_app_label, app_label = app_label, self.old_app_label
        self.database_forwards(app_label, schema_editor, from_state, to_state)
        app_label, self.old_app_label = self.old_app_label, app_label

    def describe(self):
        return "Move %s from %s" % (self.name, self.old_app_label)


class Migration(migrations.Migration):

    dependencies = [
       ('dst_app', 'XXX0_dst_app_old'),
       ('src_app', 'XXX0_src_app_old'),
    ]

    operations = [
        MoveModelFromOtherApp('MoveMe', 'src_app'),
    ]

Add dependency on XXX1_dst_app_new to XXX1_src_app_new. XXX1_src_app_new is no-op migration that is needed to make sure that future src_app migrations will be executed after XXX1_dst_app_new.

Move MoveMe from src_app/models.py to dst_app/models.py. Then run:

python manage.py migrate

That’s all!

Question 10

You can try the following (untested):

move the model from src_app to dest_app
migrate dest_app; make sure the schema migration depends on the latest src_app migration (https://docs.djangoproject.com/en/dev/topics/migrations/#migration-files)
add a data migration to dest_app, that copies all data from src_app
migrate src_app; make sure the schema migration depends on the latest (data) migration of dest_app — that is: the migration of step 3

Note that you will be copying the whole table, instead of moving it, but that way both apps don’t have to touch a table that belongs to the other app, which I think is more important.

Question 11

Lets say you are moving model TheModel from app_a to app_b.

An alternate solution is to alter the existing migrations by hand. The idea is that each time you see an operation altering TheModel in app_a’s migrations, you copy that operation to the end of app_b’s initial migration. And each time you see a reference ‘app_a.TheModel’ in app_a’s migrations, you change it to ‘app_b.TheModel’.

I just did this for an existing project, where I wanted to extract a certain model to an reusable app. The procedure went smoothly. I guess things would be much harder if there were references from app_b to app_a. Also, I had a manually defined Meta.db_table for my model which might have helped.

Notably you will end up with altered migration history. This doesn’t matter, even if you have a database with the original migrations applied. If both the original and the rewritten migrations end up with the same database schema, then such rewrite should be OK.

Question 12

change the names of old models to ‘model_name_old’
makemigrations
make new models named ‘model_name_new’ with identical relationships on the related models (eg. user model now has user.blog_old and user.blog_new)
makemigrations
write a custom migration that migrates all the data to the new model tables
test the hell out of these migrations by comparing backups with new db copies before and after running the migrations
when all is satisfactory, delete the old models
makemigrations
change the new models to the correct name ‘model_name_new’ -> ‘model_name’
test the whole slew of migrations on a staging server
take your production site down for a few minutes in order to run all migrations without users interfering

Do this individually for each model that needs to be moved. I wouldn’t suggest doing what the other answer says by changing to integers and back to foreign keys There is a chance that new foreign keys will be different and rows may have different IDs after the migrations and I didn’t want to run any risk of mismatching ids when switching back to foreign keys.

Python 实用宝典

如何在两个Django应用之间移动模型（Django 1.7）

问题：如何在两个Django应用之间移动模型（Django 1.7）

回答 0

回答 1

从旧应用中删除

添加到新应用

Remove from old app

Add to new app

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