table.foreign_keys now handles compound foreign keys, refs #594

This is a breaking change, since the return type is now a dataclass
and not a namedtuple.

docs/changelog.rst

@@ -4,6 +4,15 @@
  Changelog
 ===========
 
+.. _v_unreleased:
+
+Unreleased
+----------
+
+Breaking changes:
+
+- ``table.foreign_keys`` now returns ``ForeignKey`` objects that are dataclasses rather than ``namedtuple`` instances, so they can no longer be unpacked or indexed as ``(table, column, other_table, other_column)`` tuples - access their fields by name instead. Compound (multi-column) foreign keys are now represented as a single ``ForeignKey`` with ``is_compound=True`` and populated ``columns``/``other_columns`` lists, where ``column`` and ``other_column`` are ``None``. Previously they were returned as one ``ForeignKey`` per column, misleadingly suggesting several independent foreign keys. See :ref:`upgrading_3_to_4` for details. (:issue:`594`)
+
 .. _v4_0rc2:
 
 4.0rc2 (2026-07-04)

docs/python-api.rst

@@ -2204,17 +2204,35 @@ Almost all SQLite tables have a ``rowid`` column, but a table with no explicitly
 .foreign_keys
 -------------
 
-The ``.foreign_keys`` property returns any foreign key relationships for the table, as a list of ``ForeignKey(table, column, other_table, other_column)`` named tuples. It is not available on views.
+The ``.foreign_keys`` property returns any foreign key relationships for the table, as a list of ``ForeignKey`` objects. It is not available on views.
+
+Each ``ForeignKey`` has the following attributes:
+
+``table``
+    The table the foreign key is defined on.
+``column``
+    The column on this table, or ``None`` for a compound foreign key.
+``other_table``
+    The table being referenced.
+``other_column``
+    The referenced column, or ``None`` for a compound foreign key.
+``columns``
+    A list of the columns on this table, always populated (a single-item list for single-column foreign keys).
+``other_columns``
+    A list of the referenced columns.
+``is_compound``
+    ``True`` if this is a compound (multi-column) foreign key.
+
+``ForeignKey`` was a ``namedtuple`` prior to sqlite-utils 4.0. It is now a dataclass and can no longer be unpacked or indexed as a tuple - access its fields by name instead. See :ref:`upgrading_3_to_4` for details.
 
 ::
 
     >>> db.table("Street_Tree_List").foreign_keys
-    [ForeignKey(table='Street_Tree_List', column='qLegalStatus', other_table='qLegalStatus', other_column='id'),
-     ForeignKey(table='Street_Tree_List', column='qCareAssistant', other_table='qCareAssistant', other_column='id'),
-     ForeignKey(table='Street_Tree_List', column='qSiteInfo', other_table='qSiteInfo', other_column='id'),
-     ForeignKey(table='Street_Tree_List', column='qSpecies', other_table='qSpecies', other_column='id'),
-     ForeignKey(table='Street_Tree_List', column='qCaretaker', other_table='qCaretaker', other_column='id'),
-     ForeignKey(table='Street_Tree_List', column='PlantType', other_table='PlantType', other_column='id')]
+    [ForeignKey(table='Street_Tree_List', column='qLegalStatus', other_table='qLegalStatus', other_column='id', columns=['qLegalStatus'], other_columns=['id'], is_compound=False),
+     ForeignKey(table='Street_Tree_List', column='qCareAssistant', other_table='qCareAssistant', other_column='id', columns=['qCareAssistant'], other_columns=['id'], is_compound=False),
+     ...]
+
+Compound foreign keys - defined with ``FOREIGN KEY (col_a, col_b) REFERENCES other(col_a, col_b)`` - are returned as a single ``ForeignKey`` with ``is_compound=True``, ``column`` and ``other_column`` set to ``None``, and the participating columns available in the ``columns`` and ``other_columns`` lists.
 
 .. _python_api_introspection_schema:
 

docs/upgrading.rst

@@ -79,6 +79,22 @@ Python API changes
 
 **View.enable_fts() has been removed.** The ``View`` class previously had an ``enable_fts()`` method that existed only to raise ``NotImplementedError`` - full-text search is not supported for views. Calling it now raises ``AttributeError`` like any other missing method.
 
+**ForeignKey is now a dataclass, not a namedtuple.** The ``ForeignKey`` objects returned by ``table.foreign_keys`` gained three new fields - ``columns``, ``other_columns`` and ``is_compound`` - so that compound (multi-column) foreign keys can be represented as a single object. To make room for those fields cleanly ``ForeignKey`` is now a dataclass rather than a ``namedtuple``, so it can no longer be unpacked or indexed as a tuple. Access its fields by name instead:
+
+.. code-block:: python
+
+    # 3.x - tuple unpacking, no longer works:
+    for table, column, other_table, other_column in db["courses"].foreign_keys:
+        ...
+
+    # 4.0 - access fields by name:
+    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.
+
+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`` lists instead. Single-column foreign keys are unaffected apart from the class change: ``column``/``other_column`` behave as before and ``columns``/``other_columns`` are single-item lists.
+
 **Validation errors raise ValueError.** Invalid arguments to Python API methods - for example ``create_table()`` with no columns, or ``ignore=True`` together with ``replace=True`` - now raise ``ValueError``. They previously raised ``AssertionError`` from bare ``assert`` statements, which were silently skipped under ``python -O``.
 
 **Transaction behavior is now well-defined.** 4.0 introduces the :ref:`db.atomic() <python_api_atomic>` context manager and uses it consistently for every write operation - the full model is described in :ref:`python_api_transactions`. Changes you may notice: