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Transform now refuses to run inside a transaction if destructive foreign keys exist (#795)

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* Transform now refuses to run inside a transaction if destructive foreign keys exist

Closes #794

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014StVTWQJpFhfZJK2CYVBwv
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Simon Willison 2026-07-12 08:43:51 -07:00 • committed by GitHub
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3 changed files with 190 additions and 2 deletions
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35
sqlite_utils/db.py
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@ -2522,6 +2522,11 @@ class Table(Queryable):
See :ref:`python_api_transform` for full details.
Raises :py:class:`sqlite_utils.db.TransactionError` if called while a
transaction is open with ``PRAGMA foreign_keys`` enabled and the table
is referenced by foreign keys with destructive ``ON DELETE`` actions -
see :ref:`python_api_transform_foreign_keys_transactions`.
:param types: Columns that should have their type changed, for example ``{"weight": float}``
:param rename: Columns to rename, for example ``{"headline": "title"}``
:param drop: Columns to drop
@ -2566,6 +2571,36 @@ class Table(Queryable):
should_defer_foreign_keys = (
pragma_foreign_keys_was_on and already_in_transaction
)
if should_defer_foreign_keys:
# PRAGMA foreign_keys is a no-op inside a transaction, and
# defer_foreign_keys only defers violation checks, not ON DELETE
# actions - so dropping the old table would still fire destructive
# actions on any tables that reference it. Refuse rather than
# silently modify or delete those rows.
destructive_fks = [
(table.name, fk)
for table in self.db.tables
for fk in table.foreign_keys
if fk.other_table == self.name
and fk.on_delete in ("CASCADE", "SET NULL", "SET DEFAULT")
]
if destructive_fks:
raise TransactionError(
"Cannot transform table {table} while a transaction is open: "
"PRAGMA foreign_keys cannot be changed inside a transaction, "
"and the table is referenced by foreign keys with ON DELETE "
"actions that would fire when the old table is dropped: "
"{fks}. Call transform() outside of the transaction, or "
'execute "PRAGMA foreign_keys = off" before opening it.'.format(
table=self.name,
fks=", ".join(
"{}.{} (ON DELETE {})".format(
table_name, ", ".join(fk.columns), fk.on_delete
)
for table_name, fk in destructive_fks
),
)
)
defer_foreign_keys_was_on = False
try:
if should_disable_foreign_keys: