infi.clickhouse_orm/docs/schema_migrations.md

Schema 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.

---

<< Table Engines | Table of Contents | System Models >>