How to change the length of a Primary Key field in Alembic?

Question

I'm trying to change the length of a Primary Key field from 3 to 6.

Model:

class Server(db.Model):
    country_code = db.Column(db.String(6), primary_key=True)

Migration:

def upgrade():
    op.alter_column('server', 'country_code',
               existing_type=mysql.VARCHAR(length=3),
               type_=sa.String(length=6))

However I'm getting this error message, which I don't quite understand, why it thinks that I'm changing it to null.

_mysql_exceptions.DataError: (1171, 'All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead')

Answer 1

You need to remove the primary key property of the column before you can change it's datatype.

def upgrade():
    # Drop primary key constraint.
    op.execute('ALTER TABLE user DROP PRIMARY KEY')

    # Change type of the primary key column.
    op.alter_column('server', 'country_code',
                    existing_type=mysql.VARCHAR(length=3),
                    type_=sa.String(length=6))

    # Re-create the primary key constraint.
    op.create_primary_key(None, 'server', ['country_code'])