infi.clickhouse_orm/MIGRATIONS.rst
2016-07-11 13:04:48 +03:00

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Migrations
==========
Over time, the ORM models in your application may change. Migrations provide a way to modify the database
tables according to the changes in your models, without writing raw SQL.
The migrations that were applied to the database are recorded in the ``infi_clickhouse_orm_migrations`` table,
so migrating the database will only apply any missing migrations.
Writing Migrations
------------------
To write migrations, create a Python package. Then create a python file for the initial migration. The migration
files must begin with a four-digit number, and will be applied in sequence. For example::
analytics
|
+-- analytics_migrations
|
+-- __init__.py
|
+-- 0001_initial.py
|
+-- 0002_add_user_agents_table.py
Each migration file is expected to contain a list of ``operations``, for example::
from infi.clickhouse_orm import migrations
from analytics import models
operations = [
migrations.CreateTable(models.Visits),
migrations.CreateTable(models.Visitors)
]
The following operations are supported:
**CreateTable**
A migration operation that creates a table for a given model class.
**DropTable**
A migration operation that drops the table of a given model class.
**AlterTable**
A migration operation that compares the table of a given model class to
the model's fields, and alters the table to match the model. The operation can:
- add new columns
- drop obsolete columns
- modify column types
Default values are not altered by this operation.
Running Migrations
------------------
To migrate a database, create a ``Database`` instance and call its ``migrate`` method with the package
name containing your migrations::
Database('analytics_db').migrate('analytics.analytics_migrations')
Note that you may have more than one migrations package.