Alembic: How to add unique constraint to existing column - sqlalchemy

Question

I have a table 'test' having a column 'Name' with no constraints. I need to ALTER this column by giving it a UNIQUE constraint. How should I do it?
Should I use op.alter_column('???') or create_unique_constraint('???')?
Isn't create_unique_constraint for new column and not for existing one?

Answer 1

To add, you'd need:
https://alembic.sqlalchemy.org/en/latest/ops.html#alembic.operations.Operations.create_unique_constraint
from alembic import op
op.create_unique_constraint('uq_user_name', 'user', ['name'], schema='my_schema')
To drop, you'd need:
https://alembic.sqlalchemy.org/en/latest/ops.html#alembic.operations.Operations.drop_constraint
op.drop_constraint('uq_user_name', 'user', schema='my_schema')

Answer 2

Note: SQLAlchemy Migrations
Updated = Version: 0.7.3
to add unique constraints use create() on UniqueConstraint
to remove unique contraints use drop() on UniqueConstraint
Create a migration script. The script can be created in 2 ways.
# create manage.py
migrate manage manage.py --repository=migrations --url=postgresql://<user>:<password>#localhost:5432/<db_name>
# create script file
python manage.py script "Add Unique Contraints"
Or if you don't want to create manage.py then use the below commands
migrate script --repository=migrations --url=postgresql://<user>:<password?#localhost:5432/<db_name> "Add Unique Contraint"
it will create 00x_Add_Unique_Constraints.py
File: 00x_Add_Unique_Constraints.py
from migrate import UniqueConstraint
from sqlalchemy import MetaData, Table
def upgrade(migrate_engine):
# Upgrade operations go here. Don't create your own engine; bind
# migrate_engine to your metadata
# Table Name: user_table
# Column Name: first_name
metadata = MetaData(bind=migrate_engine)
user_table = Table('user_table', metadata, autoload=True)
UniqueConstraint(user_table.c.first_name, table=user_table).create()
def downgrade(migrate_engine):
# Operations to reverse the above upgrade go here.
# Table Name: user_table
# Column Name: first_name
metadata = MetaData(bind=migrate_engine)
user_table = Table('user_table', metadata, autoload=True)
UniqueConstraint(user_table.c.first_name, table=user_table).drop()

Answer 3

Following Mario Ruggier's answer,
I tried using his example code for my MySQL database and I didn't use the schema arguments because my database didn't have a schema.
I used:
from alembic import op
op.create_unique_constraint('uq_user_name', 'user', ['name'])
to drop the unique constraint, and
op.drop_constraint(constraint_name='uq_user_name', table_name='user', type_='unique')
Notice the different that I used a third argument, type_='unique', because without it, MySQL would return an error message that states something like
No generic 'DROP CONSTRAINT' in MySQL - please specify constraint type ...