Migrations and Schema Changes

Expand/contract migrations, data migrations and zero-downtime habits.

• Advanced
• 14 min read
• 3 objectives

Before this lessonLesson 13: The Flask CLI and Shell

Alembic (via Flask-Migrate) is how schema changes reach production. The unsafe habit is adding a non-null column with no default to a live table.

Expand, then contract

• Expand: add a nullable column, deploy code that writes to both old and new, backfill.
• Migrate reads: deploy code that reads the new column.
• Contract: drop the old column in a later release.

flask db migrate -m "add posts.excerpt nullable"
# review the generated file, then:
flask db upgrade

A data migration

def upgrade():
    op.add_column("post", sa.Column("excerpt", sa.String(200), nullable=True))
    conn = op.get_bind()
    conn.execute(sa.text("UPDATE post SET excerpt = substr(body, 1, 160) WHERE excerpt IS NULL"))

def downgrade():
    op.drop_column("post", "excerpt")