SchemaEditor

class BaseDatabaseSchemaEditor

Django のマイグレーションシステムは2つの部分に分かれています。データベースに対してどのような操作を実行するべきかを計算してその結果を保管するロジックの部分　(django.db.migrations)と、「モデルを作成する」や「フィールドを削除する」といった操作を SQL に変換する、データベースの抽象レイヤーの部分です。後者の仕事を担当するのが、 SchemaEditor です。

Django を使っている普通の開発者が SchemaEditor を直接扱うことはほとんどありませんが、自前のマイグレーションシステムを実装したい場合や、より高度なことが必要になった場合には、SQL を書くのではなく、こちらを選びましょう。

Django のデータベースのバックエンドは、それぞれ対応するバージョンの SchemaEditor を提供しており、好きなときに connection.schema_editor() コンテキストマネージャを使ってアクセスできます。

with connection.schema_editor() as schema_editor:
    schema_editor.delete_model(MyModel)

It must be used via the context manager as this allows it to manage things like transactions and deferred SQL (like creating ForeignKey constraints).

It exposes all possible operations as methods, that should be called in the order you wish changes to be applied. Some possible operations or types of change are not possible on all databases - for example, MyISAM does not support foreign key constraints.

If you are writing or maintaining a third-party database backend for Django, you will need to provide a SchemaEditor implementation in order to work with Django's migration functionality - however, as long as your database is relatively standard in its use of SQL and relational design, you should be able to subclass one of the built-in Django SchemaEditor classes and tweak the syntax a little.

メソッド

execute()

BaseDatabaseSchemaEditor.execute(sql, params=())

Executes the SQL statement passed in, with parameters if supplied. This is a wrapper around the normal database cursors that allows capture of the SQL to a .sql file if the user wishes.

create_model()

BaseDatabaseSchemaEditor.create_model(model)

Creates a new table in the database for the provided model, along with any unique constraints or indexes it requires.

delete_model()

BaseDatabaseSchemaEditor.delete_model(model)

Drops the model's table in the database along with any unique constraints or indexes it has.

add_index()

BaseDatabaseSchemaEditor.add_index(model, index)

Adds index to model’s table.

remove_index()

BaseDatabaseSchemaEditor.remove_index(model, index)

Removes index from model’s table.

add_constraint()

BaseDatabaseSchemaEditor.add_constraint(model, constraint)

Adds constraint to model's table.

remove_constraint()

BaseDatabaseSchemaEditor.remove_constraint(model, constraint)

Removes constraint from model's table.

alter_unique_together()

BaseDatabaseSchemaEditor.alter_unique_together(model, old_unique_together, new_unique_together)

Changes a model's unique_together value; this will add or remove unique constraints from the model's table until they match the new value.

alter_index_together()

BaseDatabaseSchemaEditor.alter_index_together(model, old_index_together, new_index_together)

Changes a model's index_together value; this will add or remove indexes from the model's table until they match the new value.

alter_db_table()

BaseDatabaseSchemaEditor.alter_db_table(model, old_db_table, new_db_table)

Renames the model's table from old_db_table to new_db_table.

alter_db_tablespace()

BaseDatabaseSchemaEditor.alter_db_tablespace(model, old_db_tablespace, new_db_tablespace)

Moves the model's table from one tablespace to another.

add_field()

BaseDatabaseSchemaEditor.add_field(model, field)

Adds a column (or sometimes multiple) to the model's table to represent the field. This will also add indexes or a unique constraint if the field has db_index=True or unique=True.

If the field is a ManyToManyField without a value for through, instead of creating a column, it will make a table to represent the relationship. If through is provided, it is a no-op.

If the field is a ForeignKey, this will also add the foreign key constraint to the column.

remove_field()

BaseDatabaseSchemaEditor.remove_field(model, field)

Removes the column(s) representing the field from the model's table, along with any unique constraints, foreign key constraints, or indexes caused by that field.

If the field is a ManyToManyField without a value for through, it will remove the table created to track the relationship. If through is provided, it is a no-op.

alter_field()

BaseDatabaseSchemaEditor.alter_field(model, old_field, new_field, strict=False)

This transforms the field on the model from the old field to the new one. This includes changing the name of the column (the db_column attribute), changing the type of the field (if the field class changes), changing the NULL status of the field, adding or removing field-only unique constraints and indexes, changing primary key, and changing the destination of ForeignKey constraints.

The most common transformation this cannot do is transforming a ManyToManyField into a normal Field or vice-versa; Django cannot do this without losing data, and so it will refuse to do it. Instead, remove_field() and add_field() should be called separately.

If the database has the supports_combined_alters, Django will try and do as many of these in a single database call as possible; otherwise, it will issue a separate ALTER statement for each change, but will not issue ALTERs where no change is required.

属性

特に言及されない限り、すべての属性は読み取り専用です。

connection

SchemaEditor.connection

データベースへの connection オブジェクト。connection の便利な属性に、現在アクセスしているデータベースの名前を特定するのに使える alias があります。

特に、この属性は、複数のデータベースに対するマイグレーション を行っているときに役に立ちます。

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