openstack/neutron
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neutron/doc/source/devref/db_layer.rst
Ann Kamyshnikova 4c6b565371 Add some useful notes in devref/db_layer.rst 
Added information about difference between default and server_default
parameters for columns.

This was suggested in comments to I466202ac8c378f1e19e42ec2880d0c8fbc35fd28.

Related-bug: #1295539

Change-Id: I605ab8a4f3af80497fb8ad21d43fa13d8d533325
2015-03-25 12:38:24 +03:00

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Neutron Database Layer

This section contains some common information that will be useful for developers that need to do some db changes.

Difference between 'default' and 'server_default' parameters for columns

For columns it is possible to set 'default' or 'server_default'. What is the difference between them and why should they be used?

The explanation is quite simple:

  • default - the default value that SQLAlchemy will specify in queries for creating instances of a given model;
  • server_default - the default value for a column that SQLAlchemy will specify in DDL.

Summarizing, 'default' is useless in migrations and only 'server_default' should be used. For synchronizing migrations with models server_default parameter should also be added in model. If default value in database is not needed, 'server_default' should not be used. The declarative approach can be bypassed (i.e. 'default' may be omitted in the model) if default is enforced through business logic.

Tests to verify that database migrations and models are in sync

neutron.tests.functional.db.test_migrations

_TestModelsMigrations