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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. If the table already exists, the operation does nothing.

In case the model class is a BufferModel, the operation first creates the underlying on-disk table, and then creates the buffer table.

DropTable

A migration operation that drops the table of a given model class. If the table does not exist, the operation does nothing.

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.

AlterTableWithBuffer

A compound migration operation for altering a buffer table and its underlying on-disk table. The buffer table is dropped, the on-disk table is altered, and then the buffer table is re-created. This is the procedure recommended in the ClickHouse documentation for handling scenarios in which the underlying table needs to be modified.

Applying this migration operation to a regular table has the same effect as an AlterTable operation.

AlterConstraints

A migration operation that adds new constraints from the model to the database table, and drops obsolete ones. Constraints are identified by their names, so a change in an existing constraint will not be detected unless its name was changed too. ClickHouse does not check that the constraints hold for existing data in the table.

RunPython

A migration operation that runs a Python function. The function receives the Database instance to operate on.

def forward(database):
    database.insert([
         TestModel(field=1)
    ])

operations = [
    migrations.RunPython(forward),
]

RunSQL

A migration operation that runs raw SQL statements. It expects a string containing an SQL statements, or a list of statements.

Example:

operations = [
    RunSQL('INSERT INTO `test_table` (field) VALUES (1)'),
    RunSQL([
      'INSERT INTO `test_table` (field) VALUES (2)',
      'INSERT INTO `test_table` (field) VALUES (3)'
     ])
]

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.

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