SQLAlchemy-Migrate or Alembic deletes data when renaming model

Question

I am using SQLAlchemy-Migrate to manage migrations for my PostgreSQL database. I changed the __tablename__ for a model, and running the migration changed the name in the database, but all the rows in the table were deleted. How can I rename a model without deleting data?

class Contract(db.Model):
    __tablename__ = 'contract'
    id = db.Column(db.Integer, primary_key=True)
    is_valid = db.Column(db.Boolean, default=IS_VALID)

I rename it from contract to contracts and get this migration:

def upgrade(migrate_engine):
    pre_meta.bind = migrate_engine
    post_meta.bind = migrate_engine
    pre_meta.tables['contract'].drop()
    post_meta.tables['contracts'].create()

It drops the old table and creates a new one. I never examined other migration scripts because they always ran without dropping the data.

Asked By: Nurjan

||

Source

Answer 1

SQLAlchemy-Migrate does not know that the table named "contract" in the database is the same as the model named "contracts" in the code. They’re different names, and it only does a simple comparison. This is why you always review the generated migration scripts to make sure they do the right thing.

From the SQLAlchemy-Migrate docs, rename a table with the rename method.

pre_meta.tables['contract'].rename('contracts')

If you’re using Alembic (or Flask-Alembic, or Flask-Migrate) instead of SQLAlchemy-Migrate, the same thing happens. Use the rename_table method.

op.rename_table('contract', 'contracts')

Answered By: davidism

Answer 2

I had a similar problem, for some readon, when using

op.rename_table("old_name", "new_name")

I kept getting

psycopg2.errors.DuplicateTable: relation "new_name" already exists

In the end, I used

def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    # op.drop_table('uploaded_resource')
    op.drop_table('sessions')
    
    op.execute('DROP TABLE IF EXISTS new_name')
    op.execute('ALTER TABLE old_name RENAME TO new_name')
    op.execute('ALTER SEQUENCE old_name_id_seq RENAME TO new_name_id_seq')
    op.execute('ALTER INDEX old_name_pkey RENAME TO new_name_pkey')
    ....

and

def downgrade():
    ...
    op.execute('ALTER TABLE new_name RENAME TO old_name')
    op.execute('ALTER SEQUENCE new_name_id_seq RENAME TO old_name_id_seq')
    op.execute('ALTER INDEX new_name_pkey RENAME TO old_name_pkey')

Which is less then ideal.

This also helped a fair bit
https://petegraham.co.uk/rename-postgres-table-with-alembic/

Answered By: user6855334

Answer 3

Just to add to @davidism’s answer, when using the op.rename_table() function, you should make sure to add the relevant downgrade code in as well.

For example:
If you have op.rename_table('table_name', 'new_table_name') inside of your upgrade() function, then you need to include op.rename_table('new_table_name', 'table_name') inside of the downgrade() function just in case anything goes wrong.

Hope this helps anyone that sees this, seeing that this is an old question already.

Answered By: KyleDev

SQLAlchemy-Migrate or Alembic deletes data when renaming model

Question:

Answers: