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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.
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 queries. It expects a string containing an SQL query, or an array of SQL-query strings.
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.