alembic.md

layout	default
title	Alembic integration

Alembic integration

SQLAlchemy's migration tool, Alembic, works with DuckDB without extra setup. duckdb_sqlalchemy registers its Alembic implementation, DuckDBImpl, when Alembic is loaded, so env.py needs no implementation class.

Configure alembic.ini

Point Alembic at a DuckDB URL:

sqlalchemy.url = duckdb:///analytics.db

env.py configuration

The env.py that alembic init creates works as is:

from alembic import context
from sqlalchemy import engine_from_config, pool


def run_migrations_online() -> None:
    config = context.config
    connectable = engine_from_config(
        config.get_section(config.config_ini_section),
        prefix="sqlalchemy.",
        poolclass=pool.NullPool,
    )

    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            target_metadata=target_metadata,
        )

        with context.begin_transaction():
            context.run_migrations()

An AlembicDuckDBImpl(DefaultImpl) class with __dialect__ = "duckdb" from earlier versions of these docs keeps working and takes precedence over DuckDBImpl. Remove it to get the DuckDB-specific comparison and rendering below, or subclass duckdb_sqlalchemy.alembic_impl.DuckDBImpl instead. Code that imports Alembic only after creating and connecting its engine can register DuckDBImpl with import duckdb_sqlalchemy.alembic_impl.

Create migrations

alembic revision --autogenerate -m "create tables"
alembic upgrade head

Autogenerate compares your models with reflected tables, columns, primary and foreign keys, unique and check constraints, indexes, and comments. Running autogenerate again right after upgrade head produces an empty migration.

DuckDBImpl also handles what the generic implementation gets wrong for DuckDB:

• Struct, Map, and Union columns, and arrays of them, render as duckdb_sqlalchemy.datatypes.Struct({...}) in the migration, and the file imports duckdb_sqlalchemy.datatypes.
• With compare_type=True (Alembic's default), a nested type whose fields changed is reported as a type change.
• A named unique constraint (UniqueConstraint("sku", name="uq_items_sku")) matches the constraint DuckDB created for the same columns. DuckDB replaces constraint names with its own (items_sku_key), so the generic comparison reported it as removed and added on every run.
• With compare_server_default=True, defaults are compared after undoing DuckDB's spelling ('18' for 18, CAST('t' AS BOOLEAN) for true), and the implicit nextval(...) default of an autoincrement primary key is ignored.
• op.alter_column(..., comment=...) emits COMMENT ON COLUMN.
• DuckDB rejects DROP COLUMN, column renames, and type or nullability changes on a table that has an index, even after DROP INDEX earlier in the same migration. op.drop_column() and op.alter_column() recreate such a table in batch mode and rebuild its remaining indexes, so autogenerated upgrades and downgrades run unchanged. Dropping a column also drops the indexes on it, as in PostgreSQL. Offline (--sql) scripts keep the plain ALTER TABLE statements.

Adding constraints to existing tables

DuckDB cannot add or drop UNIQUE, FOREIGN KEY, or CHECK constraints on an existing table, so op.create_unique_constraint(), op.create_foreign_key(), and op.create_check_constraint() fail with "No support for that ALTER TABLE option yet". Primary keys, adding and dropping columns, column type, nullability, and server default changes, column and table renames, indexes, and table and column comments work with the regular operations.

Use batch mode with recreate="always": Alembic creates a new table with the constraint, copies the rows, drops the old table, and renames the new one.

from alembic import op


def upgrade() -> None:
    with op.batch_alter_table("items", recreate="always") as batch_op:
        batch_op.create_unique_constraint("uq_items_sku", ["sku"])
        batch_op.create_check_constraint("ck_items_qty", "qty > 0")

DuckDB will not drop a table that another table's foreign key references, so batch mode fails for such a table. Declare its constraints when you create it.

Batch recreation preserves an existing implicit primary-key sequence and its position, both with reflection and with copy_from=your_table. Copying explicit ids does not create a replacement sequence starting at 1. Offline batch scripts with copy_from retain the model's existing implicit sequence name. Reflected sequence-backed keys compile as INTEGER/BIGINT plus nextval, never SERIAL.

Pass schema="analytics" (or a catalog-qualified schema) to batch_alter_table() for tables outside the default schema. Table renames, including the final rename during batch recreation, keep the table in that schema and quote the new table name as a single identifier.

Known limitations

• Renaming a table that has an index. DuckDB rejects ALTER TABLE ... RENAME TO while the table has an index. Drop the indexes in one migration, rename the table in the next, and create the indexes again.
• Dropping constraints by name. DuckDB ignores the names in your DDL, so drop a constraint by the name DuckDB reports, for example from inspect(conn).get_unique_constraints("items").
• Server default comparison compares SQL text, ignoring case, spacing, casts, and enclosing parentheses outside string literals. An expression that DuckDB rewrites in some other way is still reported as changed. Write the default the way DuckDB stores it (SELECT column_default FROM duckdb_columns()), or leave compare_server_default off, which is Alembic's default.