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.

tests/test_foreign_keys.py

@@ -0,0 +1,82 @@
+"""Tests for reading compound (multi-column) foreign keys - issue #594."""
+
+import pytest
+from sqlite_utils import Database
+
+COMPOUND_SCHEMA = """
+CREATE TABLE departments (
+    campus_name TEXT NOT NULL,
+    dept_code TEXT NOT NULL,
+    dept_name TEXT,
+    PRIMARY KEY (campus_name, dept_code)
+);
+CREATE TABLE courses (
+    course_code TEXT PRIMARY KEY,
+    course_name TEXT,
+    campus_name TEXT NOT NULL,
+    dept_code TEXT NOT NULL,
+    FOREIGN KEY (campus_name, dept_code)
+    REFERENCES departments(campus_name, dept_code)
+);
+"""
+
+
+@pytest.fixture
+def compound_db():
+    db = Database(memory=True)
+    db.executescript(COMPOUND_SCHEMA)
+    return db
+
+
+def test_compound_foreign_key(compound_db):
+    fks = compound_db["courses"].foreign_keys
+    assert len(fks) == 1
+    fk = fks[0]
+    assert fk.is_compound is True
+    assert fk.table == "courses"
+    assert fk.other_table == "departments"
+    assert fk.columns == ["campus_name", "dept_code"]
+    assert fk.other_columns == ["campus_name", "dept_code"]
+    # Scalar column/other_column can't sensibly hold a compound key
+    assert fk.column is None
+    assert fk.other_column is None
+
+
+def test_single_foreign_key_gets_columns_fields(fresh_db):
+    fresh_db["authors"].insert({"id": 1, "name": "Sally"}, pk="id")
+    fresh_db["books"].insert({"title": "Hedgehogs", "author_id": 1})
+    fresh_db["books"].add_foreign_key("author_id", "authors", "id")
+    fk = fresh_db["books"].foreign_keys[0]
+    assert fk.is_compound is False
+    assert fk.column == "author_id"
+    assert fk.other_column == "id"
+    assert fk.columns == ["author_id"]
+    assert fk.other_columns == ["id"]
+
+
+def test_foreign_key_no_longer_unpacks_as_tuple(fresh_db):
+    # Clean break in 4.0: ForeignKey is a dataclass, not a namedtuple, so the
+    # old tuple unpacking and indexing patterns now fail hard.
+    fresh_db["authors"].insert({"id": 1, "name": "Sally"}, pk="id")
+    fresh_db["books"].insert({"title": "Hedgehogs", "author_id": 1})
+    fresh_db["books"].add_foreign_key("author_id", "authors", "id")
+    fk = fresh_db["books"].foreign_keys[0]
+    with pytest.raises(TypeError):
+        table, column, other_table, other_column = fk
+    with pytest.raises(TypeError):
+        fk[0]
+
+
+def test_foreign_keys_are_sortable(fresh_db):
+    fresh_db["authors"].insert({"id": 1, "name": "Sally"}, pk="id")
+    fresh_db["categories"].insert({"id": 1, "name": "Wildlife"}, pk="id")
+    fresh_db["books"].insert({"title": "Hedgehogs", "author_id": 1, "category_id": 1})
+    fresh_db.add_foreign_keys(
+        [
+            ("books", "author_id", "authors", "id"),
+            ("books", "category_id", "categories", "id"),
+        ]
+    )
+    fks = sorted(fresh_db["books"].foreign_keys)
+    assert fks[0].column == "author_id"
+    assert fks[1].column == "category_id"