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112 lines
3.7 KiB
Markdown
112 lines
3.7 KiB
Markdown
Schema Migrations
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=================
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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.
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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.
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Writing Migrations
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------------------
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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:
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analytics
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+-- analytics_migrations
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+-- __init__.py
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+-- 0001_initial.py
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+-- 0002_add_user_agents_table.py
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Each migration file is expected to contain a list of `operations`, for example:
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from infi.clickhouse_orm import migrations
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from analytics import models
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operations = [
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migrations.CreateTable(models.Visits),
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migrations.CreateTable(models.Visitors)
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]
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The following operations are supported:
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### CreateTable
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A migration operation that creates a table for a given model class. If the table already exists, the operation does nothing.
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In case the model class is a `BufferModel`, the operation first creates the underlying on-disk table, and then creates the buffer table.
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### DropTable
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A migration operation that drops the table of a given model class. If the table does not exist, the operation does nothing.
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### AlterTable
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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:
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- add new columns
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- drop obsolete columns
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- modify column types
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Default values are not altered by this operation.
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### AlterTableWithBuffer
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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.
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Applying this migration operation to a regular table has the same effect as an `AlterTable` operation.
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### AlterConstraints
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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.
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### RunPython
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A migration operation that runs a Python function. The function receives the `Database` instance to operate on.
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def forward(database):
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database.insert([
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TestModel(field=1)
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])
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operations = [
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migrations.RunPython(forward),
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]
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### RunSQL
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A migration operation that runs raw SQL statements. It expects a string containing an SQL statements, or a list of statements.
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Example:
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operations = [
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RunSQL('INSERT INTO `test_table` (field) VALUES (1)'),
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RunSQL([
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'INSERT INTO `test_table` (field) VALUES (2)',
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'INSERT INTO `test_table` (field) VALUES (3)'
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])
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]
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Running Migrations
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------------------
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To migrate a database, create a `Database` instance and call its `migrate` method with the package name containing your migrations:
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Database('analytics_db').migrate('analytics.analytics_migrations')
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Note that you may have more than one migrations package.
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---
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[<< Table Engines](table_engines.md) | [Table of Contents](toc.md) | [System Models >>](system_models.md) |