Flask-Migrate (SQLAlchemy) update length of String Field

I wanted to update the length of my string field model. But when I run flask db migrate it says:

"INFO [alembic.env] No changes in schema detected."

So here's how I updated my length of a string field in my SQLALchemy model (I used postgres for the database)

• Create an empty migration file flask db revision -m "your migration message here". it created a new migration file with this content:

"""your migration message here

Revision ID: 24822ca23a29
Revises: c48c50454932
Create Date: 2023-07-20 23:37:28.918613

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = '24822ca23a29'
down_revision = 'c48c50454932'
branch_labels = None
depends_on = None


def upgrade():
    pass


def downgrade():
    pass

according to that file, if you run flask db current, it will return the down_revision value: c48c50454932.
And if you want to implement this specific migration file, you need to run flask db upgrade 24822ca23a29 which is the value of revision

• Now, update the upgrade method with your expected string field properties. I wanted to update field my_field in tablename table in my database. The existing length was 20, and I wanted to make it 50. Null value is not allowed in my case.

def upgrade():
    op.alter_column('tablename', 'my_field',
                    existing_type=sa.String(length=20),
                    type_=sa.String(length=50),
                    existing_nullable=False)

• Always use downgrade method for the fallback just in case the migration doesn't work as expected.

basically, I just wanted to make the length back to the previous value. So I switched the existing_type and type_ value

def downgrade():
    op.alter_column('tablename', 'my_field',
                    existing_type=sa.String(length=50),
                    type_=sa.String(length=20),
                    existing_nullable=False)

• Run the flask db upgrade to implement this latest migration file in your database schema and run flask db downgrade to get the previous database schema.