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Added a "Writing migrations" how-to.
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@ -26,6 +26,7 @@ you quickly accomplish common tasks.
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static-files/index
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static-files/deployment
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windows
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writing-migrations
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.. seealso::
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69
docs/howto/writing-migrations.txt
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69
docs/howto/writing-migrations.txt
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@ -0,0 +1,69 @@
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===========================
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Writing database migrations
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===========================
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This document explains how to structure and write database migrations for
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different scenarios you might encounter. For introductory material on
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migrations, see :doc:`the topic guide </topics/migrations>`.
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.. _data-migrations-and-multiple-databases:
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Data migrations and multiple databases
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When using multiple databases, you may need to figure out whether or not to
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run a migration against a particular database. For example, you may want to
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**only** run a migration on a particular database.
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In order to do that you can check the database connection's alias inside a
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``RunPython`` operation by looking at the ``schema_editor.connection.alias``
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attribute::
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from django.db import migrations
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def forwards(apps, schema_editor):
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if not schema_editor.connection.alias == 'default':
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return
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# Your migration code goes here
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class Migration(migrations.Migration):
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dependencies = [
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# Dependencies to other migrations
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]
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operations = [
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migrations.RunPython(forwards),
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]
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.. versionadded:: 1.8
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You can also provide hints that will be passed to the :meth:`allow_migrate()`
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method of database routers as ``**hints``:
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.. snippet::
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:filename: myapp/dbrouters.py
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class MyRouter(object):
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def allow_migrate(self, db, model, **hints):
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if 'target_db' in hints:
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return db == hints['target_db']
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return True
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Then, to leverage this in your migrations, do the following::
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from django.db import migrations
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def forwards(apps, schema_editor):
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# Your migration code goes here
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class Migration(migrations.Migration):
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dependencies = [
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# Dependencies to other migrations
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]
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operations = [
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migrations.RunPython(forwards, hints={'target_db': 'default'}),
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]
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@ -76,7 +76,8 @@ manipulating the data of your Web application. Learn more about it below:
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* **Migrations:**
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:doc:`Introduction to Migrations<topics/migrations>` |
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:doc:`Operations reference <ref/migration-operations>` |
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:doc:`SchemaEditor <ref/schema-editor>`
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:doc:`SchemaEditor <ref/schema-editor>` |
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:doc:`Writing migrations <howto/writing-migrations>`
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* **Advanced:**
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:doc:`Managers <topics/db/managers>` |
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@ -476,74 +476,13 @@ You can pass a second callable to
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want executed when migrating backwards. If this callable is omitted, migrating
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backwards will raise an exception.
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.. _data-migrations-and-multiple-databases:
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Data migrations and multiple databases
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When using multiple databases, you may need to figure out whether or not to
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run a migration against a particular database. For example, you may want to
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**only** run a migration on a particular database.
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In order to do that you can check the database connection's alias inside a
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``RunPython`` operation by looking at the ``schema_editor.connection.alias``
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attribute::
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from django.db import migrations
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def forwards(apps, schema_editor):
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if not schema_editor.connection.alias == 'default':
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return
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# Your migration code goes here
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class Migration(migrations.Migration):
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dependencies = [
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# Dependencies to other migrations
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]
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operations = [
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migrations.RunPython(forwards),
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]
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.. versionadded:: 1.8
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You can also provide hints that will be passed to the :meth:`allow_migrate()`
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method of database routers as ``**hints``:
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.. snippet::
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:filename: myapp/dbrouters.py
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class MyRouter(object):
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def allow_migrate(self, db, model, **hints):
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if 'target_db' in hints:
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return db == hints['target_db']
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return True
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Then, to leverage this in your migrations, do the following::
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from django.db import migrations
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def forwards(apps, schema_editor):
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# Your migration code goes here
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class Migration(migrations.Migration):
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dependencies = [
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# Dependencies to other migrations
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]
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operations = [
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migrations.RunPython(forwards, hints={'target_db': 'default'}),
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]
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More advanced migrations
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~~~~~~~~~~~~~~~~~~~~~~~~
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If you're interested in the more advanced migration operations, or want
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to be able to write your own, see the :doc:`migration operations reference
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</ref/migration-operations>`.
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</ref/migration-operations>` and the "how-to" on :doc:`writing migrations
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</howto/writing-migrations>`.
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.. _migration-squashing:
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@ -806,3 +745,7 @@ More information is available in the
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:doc:`The Migrations Operations Reference </ref/migration-operations>`
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Covers the schema operations API, special operations, and writing your
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own operations.
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:doc:`The Writing Migrations "how-to" </howto/writing-migrations>`
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Explains how to structure and write database migrations for different
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scenarios you might encounter.
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