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.