From 26714aa5c7b86bc94c4f66cb070ec4a5de189684 Mon Sep 17 00:00:00 2001
From: "Dr. Patrick Urbanke" <patrick@getml.com>
Date: Thu, 1 Jan 2026 12:51:33 +0100
Subject: [PATCH] Documented support for multiple primary keys; resolves #107

---
 docs/primary_key.md | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/docs/primary_key.md b/docs/primary_key.md
index 9072c4b6..b0644383 100644
--- a/docs/primary_key.md
+++ b/docs/primary_key.md
@@ -27,6 +27,31 @@ CREATE TABLE IF NOT EXISTS "People"(
 );
 ```
 
+### Multiple Primary Keys
+
+You can define multiple primary keys by using `sqlgen::PrimaryKey` on multiple fields. This will create a composite primary key.
+
+```cpp
+struct Order {
+    sqlgen::PrimaryKey<int> order_id;
+    sqlgen::PrimaryKey<int> product_id;
+    int quantity;
+};
+```
+
+Now the generated SQL schema will look like this:
+
+```sql
+CREATE TABLE IF NOT EXISTS "Order"(
+    "order_id"   INTEGER NOT NULL, 
+    "product_id" INTEGER NOT NULL, 
+    "quantity"   INTEGER NOT NULL,
+    PRIMARY KEY("order_id", "product_id")
+);
+```
+
+Note that this is not supported in SQLite, as it does not support composite primary keys.
+
 ### Auto-incrementing Primary Keys
 
 You can define an auto-incrementing primary key by providing `sqlgen::auto_incr` as the second template argument to `sqlgen::PrimaryKey`. The underlying type of an auto-incrementing primary key must be an integral type.