Flask-migrate and changing column type

Just need to add compare_type=True in the Migrate constructor

from flask_migrate import Migratemigrate = Migrate(compare_type=True)app = Flask(__name__)migrate.init_app(app)

Update:

As expected, this answer is now out of date, please see https://stackoverflow.com/a/52181159/428907 for a real fix!

Original Answer:

Alembic does not recognize things like column type changes when auto-generating revisions by default. When making these more granular changes, you will need to hand-modify the migration to include these changes

e.g., in your migration file

from alembic import opimport sqlalchemy as sadef upgrade():    # ...    op.alter_column('downloads', 'size', existing_type=sa.Integer(), type_=sa.BigInteger())def downgrade():    # ...    op.alter_column('downloads', 'size', existing_type=sa.BigInteger(), type_=sa.Integer())

For details on the operations, see the operation reference

You can turn on detection of type changes by modifying your env.py and alembic.ini, as seen here

By default, Flask-migrate doesn't track the changes when type of column is changed. One of many ways to achieve this is to set

compare_type=True

Under env.py. e.g. -

context.configure(connection=connection,                      target_metadata=target_metadata,                      process_revision_directives=process_revision_directives,                      compare_type=True,                      **current_app.extensions['migrate'].configure_args)