From bfd74a35bb055a2dd28da8eabd149e4e03f30fed Mon Sep 17 00:00:00 2001
From: Simon Willison <swillison@gmail.com>
Date: Wed, 24 Jun 2026 10:43:53 -0700
Subject: [PATCH] upsert() detects existing compound primary keys

Closes #629
---
 sqlite_utils/db.py   |  5 +++++
 tests/test_upsert.py | 47 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/sqlite_utils/db.py b/sqlite_utils/db.py
index 38e95c7..ae99322 100644
--- a/sqlite_utils/db.py
+++ b/sqlite_utils/db.py
@@ -3554,6 +3554,11 @@ class Table(Queryable):
         if hash_id_columns and hash_id is None:
             hash_id = "id"
 
+        if upsert and not pk and not hash_id and self.exists():
+            existing_pks = [column.name for column in self.columns if column.is_pk]
+            if existing_pks:
+                pk = existing_pks[0] if len(existing_pks) == 1 else tuple(existing_pks)
+
         if upsert and (not pk and not hash_id):
             raise PrimaryKeyRequired("upsert() requires a pk")
 
diff --git a/tests/test_upsert.py b/tests/test_upsert.py
index de0e362..09783bb 100644
--- a/tests/test_upsert.py
+++ b/tests/test_upsert.py
@@ -49,6 +49,53 @@ def test_upsert_error_if_no_pk(fresh_db):
         table.upsert({"id": 1, "name": "Cleo"})
 
 
+def test_upsert_error_if_existing_table_has_no_pk(fresh_db):
+    table = fresh_db.create_table("table", {"id": int, "name": str})
+    with pytest.raises(PrimaryKeyRequired):
+        table.upsert({"id": 1, "name": "Cleo"})
+
+
+@pytest.mark.parametrize("use_old_upsert", (False, True))
+def test_upsert_uses_compound_pk_from_existing_table(use_old_upsert):
+    # https://github.com/simonw/sqlite-utils/issues/629
+    db = Database(memory=True, use_old_upsert=use_old_upsert)
+    db.execute("""
+        create table summary (
+            Source text,
+            Object text,
+            Category text,
+            Count integer,
+            primary key (Source, Object, Category)
+        )
+        """)
+    table = db["summary"]
+    table.upsert(
+        {
+            "Source": "Client A",
+            "Object": "Accounts",
+            "Category": "All",
+            "Count": 3,
+        }
+    )
+    assert table.last_pk == ("Client A", "Accounts", "All")
+    table.upsert(
+        {
+            "Source": "Client A",
+            "Object": "Accounts",
+            "Category": "All",
+            "Count": 4,
+        }
+    )
+    assert list(table.rows) == [
+        {
+            "Source": "Client A",
+            "Object": "Accounts",
+            "Category": "All",
+            "Count": 4,
+        }
+    ]
+
+
 def test_upsert_with_hash_id(fresh_db):
     table = fresh_db["table"]
     table.upsert({"foo": "bar"}, hash_id="pk")