Rename automatically named foreign keys with Alembic
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:date: 2017-01-02T09:41:23Z
:category: blog
:tags: mysql,development,flask,python
:url: 2017/01/02/rename-automatically-named-foreign-keys-with-alembic/
:save_as: 2017/01/02/rename-automatically-named-foreign-keys-with-alembic/index.html
:status: published
:author: Gergely Polonkai

I have recently messed up my Alembic migrations while modifying my SQLAlchemy models.  To start
with, I didn’t update the auto-generated migration files to name the indexes/foreign keys a name,
so Alembic used its own naming scheme.  This is not an actual problem until you have to modify
columns that have such constraints.  I have since fixed this problem, but first I had to find
which column references what (I had no indexes other than primary key back then, so I could go
with foreign keys only).  Here is a query I put together, mostly using `this article
<http://www.binarytides.com/list-foreign-keys-in-mysql/>`_.

.. code-block:: sql

   SELECT constraint_name,
          CONCAT(table_name, '.', column_name) AS 'foreign key',
          CONCAT(referenced_table_name, '.', referenced_column_name) AS 'references'
   FROM information_schema.key_column_usage
   WHERE referenced_table_name IS NOT NULL AND
         table_schema = 'my_app';

Now I could easily drop such constraints using

.. code-block:: python

   alembic.op.drop_constraint('users_ibfk1', 'users', type_='foreignkey')

and recreate them with

.. code-block:: python

   alembic.op.create_foreign_key('fk_user_client', 'users', 'clients', ['client_id'], ['id'])