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ForeignKey is now a frozen dataclass, restoring hashability

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The namedtuple-to-dataclass change made ForeignKey unhashable, breaking
set(table.foreign_keys) and dict-key usage that worked in 3.x. frozen=True
restores immutability and hashability. Equality and hashing cover all
compared fields including on_delete/on_update - two foreign keys differing
only in their actions are different constraints.

Refs https://github.com/simonw/sqlite-utils/issues/769#issuecomment-4900034150

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Simon Willison 2026-07-06 21:35:45 -07:00
commit 404e935b63
4 changed files with 51 additions and 8 deletions
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docs/upgrading.rst
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@ -95,7 +95,7 @@ Python API changes
for fk in db["courses"].foreign_keys:
fk.table, fk.column, fk.other_table, fk.other_column
Attempting the old unpacking or ``fk[0]`` indexing now raises ``TypeError``, so any code using those patterns will fail loudly rather than silently misbehave.
Attempting the old unpacking or ``fk[0]`` indexing now raises ``TypeError``, so any code using those patterns will fail loudly rather than silently misbehave. Like the old namedtuple, ``ForeignKey`` instances are immutable and hashable - they can be collected into sets and used as dictionary keys. Note that equality now includes the ``on_delete`` and ``on_update`` actions: a ``ForeignKey`` with ``ON DELETE CASCADE`` is not equal to one without.
Compound foreign keys - previously returned as one ``ForeignKey`` per column, misleadingly suggesting several independent single-column keys - are now returned as a single ``ForeignKey`` with ``is_compound=True``. For these the scalar ``column`` and ``other_column`` fields are ``None``; use the ``columns`` and ``other_columns`` tuples instead. Single-column foreign keys are unaffected apart from the class change: ``column``/``other_column`` behave as before and ``columns``/``other_columns`` are one-item tuples.