add_foreign_key() accepts compound keys, composite FK indexes, refs #594

table.add_foreign_key() and db.add_foreign_keys() now accept lists of
column names to create compound foreign keys:

    table.add_foreign_key(
        ["campus_name", "dept_code"], "departments"
    )

Omitting the other columns uses the compound primary key of the other
table. Duplicate detection compares the full column lists.

db.index_foreign_keys() now creates a single composite index across the
columns of a compound foreign key, rather than an index per column.

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

docs/python-api.rst

@@ -1562,6 +1562,16 @@ To ignore the case where the key already exists, use ``ignore=True``:
 
     db.table("books").add_foreign_key("author_id", "authors", "id", ignore=True)
 
+To add a compound foreign key, pass lists of columns:
+
+.. code-block:: python
+
+    db.table("courses").add_foreign_key(
+        ["campus_name", "dept_code"], "departments", ["campus_name", "dept_code"]
+    )
+
+As with single columns, omitting the other columns will use the compound primary key of the other table. ``other_table`` must always be specified for a compound foreign key.
+
 .. _python_api_add_foreign_keys:
 
 Adding multiple foreign key constraints at once
@@ -1591,6 +1601,8 @@ If you want to ensure that every foreign key column in your database has a corre
 
     db.index_foreign_keys()
 
+Compound foreign keys get a single composite index across their columns.
+
 .. _python_api_drop:
 
 Dropping a table or view