Data Migration

Data Migration is a process of keeping correct data in database after updating to new module version. For example, simple field renaming leads to data lost if you don’t have proper data migration scripts.

For Module Migration see Porting Modules

Preparing

Those migrations are between module version.

From Odoo https://github.com/odoo/odoo/blob/11.0/odoo/modules/migration.py#L53:

This class manage the migration of modules. Migrations files must be python files containing a migrate(cr, installed_version) function. Theses files must respect a directory tree structure: A ‘migrations’ folder which contains a folder by version. Version can be ‘module’ version or ‘server.module’ version (in this case, the files will only be processed by this version of the server). Python file names must start by pre or post and will be executed, respectively, before and after the module initialisation. end scripts are run after all modules have been updated.

Example:

<moduledir>
`-- migrations
    |-- 1.0
    |   |-- pre-update_table_x.py
    |   |-- pre-update_table_y.py
    |   |-- post-create_plop_records.py
    |   |-- end-cleanup.py
    |   `-- README.txt                      # not processed
    |-- 9.0.1.1                             # processed only on a 9.0 server
    |   |-- pre-delete_table_z.py
    |   `-- post-clean-data.py
    `-- foo.py                              # not processed

Execution

Migration files are just code files that don’t need to be registered anywhere. When updating an addon Odoo searching in the migrations for folders with a version in between, up to, and including the version that is in updating for. It happens before all other files were observed, so at this moment nothing is changed at your database layout. Then, if folders are found Odoo executes python files with prefix pre- in it. They should contain a defined function called migrate. This function has two arguments: database cursor and currently installed version.

After all pre-migrate functions were successfully executed, Odoo updates the module. Now, the database might be different from the previous version one. For example, if in a new version we changed the model field type, in the database this column will be changed without data preserving. Or if a field was renamed, in the new version just a new column will be created.

Then, after the module was updated, Odoo search for post-migrate files by the same algorithm and execute them.

end scripts are run after all modules have been updated.

Warning

Migration updates are not rollbacked if some errors happened later during modules updating process. So, you shall always try to update module with migration scripts on a copy first.

Example

POS Debt & Credit notebook. We need to preserve credit_product field data in the product.template model after updating to a newer version. In previous version it was boolean field, now it is a many2one field with the relation to account.journal model. Here, we, using a temporary column, calculate transfer data from boolean to many2one credit_product field.

pre-migrate.py:

def migrate(cr, version):
    # Add temporary credit product column
    cr.execute('ALTER TABLE product_template ADD temporary_credit_product int')
    # Select debt journals
    cr.execute('SELECT id FROM account_journal WHERE account_journal.debt is true')
    # Take the first journal
    journal_id = cr.fetchone()
    if journal_id:
        # set token one to all credit products
        cr.execute('UPDATE product_template SET temporary_credit_product=%s WHERE credit_product is true', journal_id)

post-migrate.py:

def migrate(cr, version):
    # update new credit_product column with the tempory one
    cr.execute('UPDATE product_template SET credit_product=temporary_credit_product')
    # Drop temporary column
    cr.execute('ALTER TABLE product_template DROP COLUMN temporary_credit_product')