Django migrations for a cloned db - django

I cloned a postgres db and added a new model to one of the apps. Our project contains many apps.
Now When I ran migrations, migrate it would fail. So I commented that model out, deleted migrations.py file from folder and ran fake migrations. Than again put that model in and ran migrations, migrate. Things were fine.
But now I deleted this model table from db manually and when would run migrations it would show model doesn't exist.
Basically I would need to again and again tweak the model, delete, update table.
So I searched for migrating from scratch. Did delete some apps from django_migrations table. But it is not working out it shows relations already existing.
This is all becoming confusing, --fake, delete, squash what to do?
Basically if I drop table django_migrations, delete migrations folder from app. Can't django automatically sync with db and understand what model exist and what don't and figure it out itself.

If you want to create a clone of a Django database, without the data, there's one table you should always get the data from: django_migrations. This is the table that holds the state of the database: which migrations have been applied and as such what models exist already.
The fact that Django tries to create one it's core models tells me you didn't have the data from django_migrations in the clone.

This solved the problem.
deleted all the data from 'django_migrations' table.
deleted all the migrations file from all migrations folder in different apps
Commented out the models whose table are still not in database
ran python manage.py makemigrations
ran python manage.py migrate --fake
Put the comments off the model
ran python manage.py makemigrations
ran python manage.py migrate
Thing is to understand what does makemigrations and migrate does. And then you can delete/update manually or created new models and migrate/clone things etc. it would work. For a new starter these commands are little confusing.
Let's says everything is empty i.e. no files in migration folder of app.
Than makemigrations makes these files which contains say sqls/syntax on how to create your table. With each change it will create a new one, with only the changes listed in that.
migrate creates actual empty tables in the database. And creates an entry in django_migrations against the app that model was part of. This entry tells it when was the latest entries from that file were applied to the database.
Scenario 1:
I deleted one table manually from db. Now django doesn't know about it. So if you would run makemigrations or migrate it would do nothing.
Scenario 2:
I deleted all entries from django_migrations, deleted migrations files from app. Tables are still in db. Now when I do makemigrations it works, but when I migrate it throws error that already existing.
Scenario 3:
Have deleted entries from django_migrations and migration files. Django has one option of migrate --fake, this tells it that tables are already existing just make entries in django_migrations table. so makemigrations and migrate --fake works now without error. But now the table which I deleted manually is still deleted, django hadn't made it. So will throw error when I try to access it.
Scenario 4:
The one I described at start. I faked the entries which were in db and then migrated the model which wasn't in db.
So once one understands what's it's doing behind the scenes many approaches could have been followed, create the table schema manually or selectively pick django_migrations or migrations file entries and delete them. But best is to delete everything and start from scratch.
It would have been best that django understands that these are tables already created, these are models I have, create the missing one, leave the existing ones. If any discrepancy show them. I see so many questions related to migrations and people are confused here and there.

Related

Delete Heroku migrations

I read advices how to delete migrations but I don't understand what I'm doing and it's not working for me.
History. One day I had an issue when I added or renamed a model fields locally. So I was tired with that issue and I deleted all migrations and migrate again. And all was OK. But I remember that I will have a big problem when I will deploy on Heroku.
So the days are gone. And now it happened. :(((
I make migrations, migrate to a server database. Pushed my code, but.. it wrote me:
relation "accounts_goal" does not exist
LINE 1: SELECT COUNT(*) AS "__count" FROM "accounts_goal"
I understand it happened because locally I have 0001 and 0002 migrations, but on the server there are 0012 and etc. migrations. I think I need to delete all old migrations on the server. But I don't know how to do that. Help me please! Thank you)
At first, do not delete a table from the database before the migration. If you change your model, then manage.py migrate will do the thing. Django cannot detect the direct change of database; only knows the change of model script (makemigrations).
Sometimes migration doesn't work for no reasons. In that case, do the following things:
Undo the change of models.py (comment, delete).
manage.py makemigrations app-name.
manage.py migrate
Change the models.py again (as you wish).
Do the migration again.
Also you can try python manage.py migrate --fake.
If there is already a table present, Django will see that the initial migration has already been applied since the table is already present with old schema and therefore not taking the new table with different schema into consideration.
In that case, you need to drop tables in the database you can use python manage.py migrate app-name zero.

Relation does not exist, in PostgreSQL, Django

So I've created a new model in Django, then executed both python manage.py makemigrations and python manage.py migrate in the right order. But then for some reason I accidentally dropped the table(relation) in PgAdmin (I know it sounds silly). So I tried deleting all the files in migrations folder but the init.py file. Then I ran the two commands above again, but could see no table in the PgAdmin. What should I do, aside from creating a table myself in PgAdmin?
Thanks in advance.
It won't work, because entry for all the migrations are already stored inside a table named django_migrations. So even if you run makemigrations after deleting all the migration files, it won't create a new one. So here is three ways you can fix it.
One: Drop DB(if data is not important)
Drop database and create a new one. Run makemigrations and migrate command.
Two: Create that table manually
If you already deleted all the migration files, you better restore them. You can use git for this: git checkout /path/to/migration/folder. Then you can manually create the table.
Three: Delete entries from django_migrations
I assumed you have deleted all the migration files. So this part covers the whole project. But #DenizKaplan has explained better way to do this.
If you are not using git, or no way to restore these files, then you can follow these steps:
Backup your database
Delete all entries from djang_migrations table
Run ./manage.py makemigrations to generate migration file
Run ./manage.py migrate <your lost app> to migrate your app(which you have lost in DB).
Run ./manage.py migrate --fake to fake the rest of apps.
There are some steps you need to check. If you have an empty output after makemigrations operations, you may need to check for django_migrations table to remove rows related to apps that you have working with. If this won't help at first place, you need check INSTALLED_APPS, maybe you may accidently delete apps. If all these not work, please give some detailed information about the error.

Django migrate didn’t launch execute some migration files

I have a Postgres database full of data. And I made several changes to my Django app models.
mange.py makemigrations worked fine and created the migration files. But manage.py migrate execute only one file. And when I launch it again it doesn’t execute the rest as if they are already applied.
I deleted the migration files that were not applied and did another makemigration but it says no changes detected.
Any ideas how to reflect the models changes on the database without losing the data ?
Thanks
Django keeps track of which migrations it has applied already, so when you run the migrate command it will execute only the migrations that Django thinks that are missing.
I deleted the migration files that were not applied and did another makemigration but it says no changes detected.
This was a bad idea, it will make your migrations inconsistent.
If you want to go back in time, instead of deleting migrations, the proper way to do this is by reverting migrations. You can use the same migrate command and specify to which migration point you want your database model to be.
Check this answer for further information about reverting migrations; django revert last migration

Django migrations : relation already exists

I have trouble with django model migrations.
I have some models in my app, and I already have some data inside.
When I added some models in my application, and I run makemigrations, the app report that there is no change.
I know that sometimes some errors came when migrate, so I delete django_migrations table in my database and run makemigrations again, and now program found my new fields.
The problem now is that if I run migrate system tell me that some tables already exist. (Which is ok and correct, because they do). I don't want to delete those tables, because I have data already inside.
I can't run migrate --fake, because program will think that I already have all the tables, which is not true.
So, I am looking for a way to tell the program : run migration, if table exist skip it. (--fake it)
Another question is why is this happening to me, that makemigrations don't recognise my changes (some cache problems,...)?
How about doing this way ?
python manage.py makemigrations
(Skip this step if you have already have migration file ready)
It will create migrations for that package lets say with a name like 0001_initial.py
Edit the file manually so that you delete all models there except that was already created in database.
Now you do a fake migration. This will sync your database with models.
python manage.py migrate --fake
Then run makemigrations again to have rest of the tables created along with a new migration file.
python manage.py makemigrations
Regarding your other question, Why makemigrations didn't recogonize your models can be because of reasons like:
Migrations for those changes are already there in some migration file.
You missed it to mention package_name in INSTALLED_APPS but i believe you did it here.
every time you make changes to your models, try these steps :
python manage.py makemigrations [your app name]
then:
python manage.py migrate
it should work fine. but remember if you have already data(rows) in your tables you should specify the default value for each one the queries.
if not, Django prompt you to specify the default value for them
or you can just try to use blank=True or null=True in your fields like below :
website = models.URLField(blank=True)
the possible cause or this is that you have another migration in the same folder starts with the same prefix... maybe you make another migration on the same table on another branch or commit so it's saved to the db with the same prefix ie: 00010_migration_from_commit_#10, 00010_migration_from_commit_#11
the solution for this is to rename the migration file like this 00011_migration_from_commit_#11
I tried to edit the related migration file and commented the part where it creates that specific column, then ran python manage.py migrate
The main problem is the existing tables that are disabling the migration of the new tables, so the solution is straight-forward:
** Try to add managed = False to the existing dB so it won't be detected by migrate
** Redo it for all existing old tables :
class Meta:
managed=False
It sometimes gets boring when we have a lot of tables in the same application but it works perfectly!

newbie difficulty using south with pycharm - DatabaseError: no such table: south_migrationhistory

I'm using sqlite3 and pycharm to learn more about django, and googled to find that south is recommended to make it easier to modify models after they have been created.
I'm trying to follow the advice on http://south.aeracode.org/docs/tutorial/part1.html#starting-off.
The most success I've had so far is to create a simple model and run syncdb before adding south to installed_apps. That way the intial tables are created and I get a chance to create a super user. (Django admin seems to fret if there are no users).
Then I add south to installed_apps, and run django_manage.py schemamigration bookmarks --initial
It seems to work fine. A new directory is created called migrations with a couple of files in it in my app folder and an encouraging message.
"Created 0001_initial.py. You can now apply this migration with: ./manage.py migrate bookmarks"
The next step - django_manage.py" migrate bookmarks generates the following error message
django.db.utils.DatabaseError: no such table: south_migrationhistory.
I thought that table would be created in the first schememigration step. What am I missing? Can anyone help?
Marg
South uses a table if its own to keep track of which migrations have been applied. Before you can apply any migrations, this must have been created, using python ./manage.py syncdb.
As well as for setting up south, you will find syncdb sometimes necessary for non-south apps in your project, such as the very common django.contrib.auth.
Note that as a convenience, you can run both in one go like this
python ./manage.py syncdb --migrate
My latest (unsuccessful) effort was the following
Create application – synch db – superuser created
Test run –admin screen shows basic tables
Add south, and syncdb from command line with manage.py syncdb – south_migrationhistory table created. Add basic vanilla model
Tried various combinations of manage.py syncdb –manage, and
schemamigration from Pycharm (if run from within pycharm a
migrations directory is created within the app
– if run from the command line the directory does not seem to be
created.)
Django admin screen shows table – but if I try to edit
the table it says that it doesn’t exist
Check database structure
using SQLite browser - table for newly created model doesn’t exist
I’m starting to think that the whole thing is not worth the time wasting hassle – maybe I’m better off just modifying the tables in SQLite browser
Answer in the similar question:
Run syncdb to add the Django and South tables to the database.