Compound primary keys now resolve in PRIMARY KEY declaration order

PRAGMA table_info sets is_pk to the 1-based position of each column
within the PRIMARY KEY, which can differ from table column order.
table.pks previously returned table column order, so an implicit
compound FOREIGN KEY ... REFERENCES other was introspected with its
referenced columns inverted, and transform() baked that inverted order
into the rewritten schema - failing with IntegrityError on valid data,
or silently reversing the constraint with foreign keys off.

table.pks, compound foreign key guessing (create, add_foreign_key) and
transform() now all use declaration order, and transform() no longer
reorders a compound PRIMARY KEY (b, a) into table column order.

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

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_foreign_keys.py

@@ -524,3 +524,57 @@ def test_add_compound_foreign_key_on_delete(courses_db):
     assert fk.is_compound is True
     assert fk.on_delete == "SET NULL"
     assert "ON DELETE SET NULL" in courses_db["courses"].schema
+
+
+def test_implicit_compound_foreign_key_resolves_pk_declaration_order(fresh_db):
+    # The other table's PRIMARY KEY declares its columns in a different
+    # order to the table's column order. SQLite resolves the implicit
+    # "REFERENCES other" using PRIMARY KEY declaration order, so the
+    # introspected other_columns must too
+    fresh_db.execute("create table other (b text, a text, primary key (a, b))")
+    fresh_db.execute(
+        "create table child (x text, y text, foreign key (x, y) references other)"
+    )
+    fk = fresh_db["child"].foreign_keys[0]
+    assert fk.other_columns == ("a", "b")
+
+
+def test_transform_implicit_compound_foreign_key_stays_valid(fresh_db):
+    # transform() rewrites the implicit FK with explicit columns - they
+    # must be in PRIMARY KEY declaration order or valid data fails the
+    # foreign key check with an IntegrityError
+    fresh_db.execute("create table other (b text, a text, primary key (a, b))")
+    fresh_db.execute(
+        "create table child (x text, y text, foreign key (x, y) references other)"
+    )
+    fresh_db.execute("PRAGMA foreign_keys = ON")
+    fresh_db["other"].insert({"a": "A", "b": "B"})
+    fresh_db["child"].insert({"x": "A", "y": "B"})
+    fresh_db["child"].transform(types={"x": str})
+    assert fresh_db["child"].foreign_keys[0].other_columns == ("a", "b")
+    # The constraint still points the right way around
+    fresh_db["child"].insert({"x": "A", "y": "B"})
+    with pytest.raises(sqlite3.IntegrityError):
+        fresh_db["child"].insert({"x": "B", "y": "A"})
+
+
+def test_create_compound_foreign_key_guesses_pk_declaration_order(fresh_db):
+    fresh_db.execute("create table other (b text, a text, primary key (a, b))")
+    fresh_db["other"].insert({"a": "A", "b": "B"})
+    fresh_db["child"].create(
+        {"id": int, "x": str, "y": str},
+        pk="id",
+        foreign_keys=[(("x", "y"), "other")],
+    )
+    assert fresh_db["child"].foreign_keys[0].other_columns == ("a", "b")
+    fresh_db.execute("PRAGMA foreign_keys = ON")
+    fresh_db["child"].insert({"id": 1, "x": "A", "y": "B"})
+    with pytest.raises(sqlite3.IntegrityError):
+        fresh_db["child"].insert({"id": 2, "x": "B", "y": "A"})
+
+
+def test_add_compound_foreign_key_guesses_pk_declaration_order(fresh_db):
+    fresh_db.execute("create table other (b text, a text, primary key (a, b))")
+    fresh_db["child"].insert({"id": 1, "x": "A", "y": "B"}, pk="id")
+    fresh_db["child"].add_foreign_key(("x", "y"), "other")
+    assert fresh_db["child"].foreign_keys[0].other_columns == ("a", "b")

tests/test_introspect.py

@@ -321,3 +321,18 @@ def test_table_default_values(fresh_db, value):
     )
     default_values = fresh_db["default_values"].default_values
     assert default_values == {"value": value}
+
+
+def test_pks_use_primary_key_declaration_order(fresh_db):
+    # PRIMARY KEY (a, b) declared against columns stored in order (b, a) -
+    # pks must follow the declaration order, which is what SQLite uses to
+    # resolve implicit foreign key references and compound pk lookups
+    fresh_db.execute("create table t (b text, a text, primary key (a, b))")
+    assert fresh_db["t"].pks == ["a", "b"]
+
+
+def test_transform_preserves_compound_pk_declaration_order(fresh_db):
+    fresh_db.execute("create table t (a text, b text, c text, primary key (b, a))")
+    fresh_db["t"].transform(drop={"c"})
+    assert fresh_db["t"].pks == ["b", "a"]
+    assert 'PRIMARY KEY ("b", "a")' in fresh_db["t"].schema