Alter Primary Key in Alembic?

I also was in the same situation: alter primary key. In my case, I had to change the primary key type from integer to string.

The primary key also had a foreign key relationship to another table. The earlier alembic migration created the foreign key constraint in the following way:

#!/usr/bin/python3

from alembic import op
import sqlalchemy as sa


def upgrade():
    op.create_table('user',
                    sa.Column('id', sa.Integer(), nullable=False),
                    sa.Column('name', sa.String(length=100), nullable=False),
                    sa.Column('username', sa.String(length=100), nullable=False),
                    sa.PrimaryKeyConstraint('id', name=op.f('pk_user')),
                    sa.UniqueConstraint('username', name=op.f('uq_user_username'))
                    )

    op.create_table('role',
                    sa.Column('id', sa.Integer, primary_key=True),
                    sa.Column('name', sa.String(100)),
                    sa.Column('description', sa.String(255)),
                    sa.PrimaryKeyConstraint('id', name=op.f('pk_role'))
                    )

    op.create_table('roles_users',
                    sa.Column('user_id', sa.Integer, nullable=True),
                    sa.Column('role_id', sa.Integer, nullable=True),
                    sa.ForeignKeyConstraint(['user_id'], ['user.id'],
                                            name=op.f('fk_roles_user_user_id_user')),
                    sa.ForeignKeyConstraint(['role_id'], ['role.id'],
                                            name=op.f('fk_roles_user_role_id_role'))
                    )

Now when changing the primary key type of the user table from Integer to String, I had to do the following:

from alembic import op
import sqlalchemy as sa


def upgrade():
    # Drop primary key constraint. Note the CASCASE clause - this deletes the foreign key constraint.
    op.execute('ALTER TABLE user DROP CONSTRAINT pk_user CASCADE')
    # Change primary key type
    op.alter_column('user', 'id', existing_type=sa.Integer, type_=sa.VARCHAR(length=25))
    op.alter_column('roles_users', 'user_id', existing_type=sa.Integer, type_=sa.VARCHAR(length=25))
    # Re-create the primary key constraint
    op.create_primary_key('pk_user', 'user', ['id'])
    # Re-create the foreign key constraint
    op.create_foreign_key('fk_roles_user_user_id_user', 'roles_users', 'user', ['user_id'], ['id'], ondelete="CASCADE")

Flask version: 0.12.1

Alembic version: 0.9.1

Python version: 3.4.4

Hope this information helps someone facing a similar problem.