Target is required on do_update

docs/src/piccolo/query_clauses/on_conflict.rst

@@ -78,6 +78,7 @@ Instead, if we want to update the ``popularity``:
     ...     Band(name="Pythonistas", popularity=1200)
     ... ).on_conflict(
     ...     action="DO UPDATE",
+    ...     target=Band.name,
     ...     values=[Band.popularity]
     ... )
 
@@ -93,8 +94,9 @@ If we fetch the data from the database, we'll see that it was updated:
 
 Using the ``target`` argument, we can specify which constraint we're concerned
 with. By specifying ``target=Band.name`` we're only concerned with the unique
-constraint for the ``band`` column. If you omit the ``target`` argument, then
-it works for all constraints on the table.
+constraint for the ``band`` column. If you omit the ``target`` argument on
+``DO NOTHING`` action, then it works for all constraints on the table. For 
+``DO UPDATE`` action, ``target`` is mandatory and must be provided.
 
 .. code-block:: python
     :emphasize-lines: 5

piccolo/query/mixins.py

@@ -769,6 +769,11 @@ def on_conflict(
         else:
             raise ValueError("Unrecognised `on conflict` action.")
 
+        if target is None and action_ == OnConflictAction.do_update:
+            raise ValueError(
+                "The `target` option must be provided with DO UPDATE."
+            )
+
         if where and action_ == OnConflictAction.do_nothing:
             raise ValueError(
                 "The `where` option can only be used with DO NOTHING."

tests/table/test_insert.py

@@ -174,6 +174,27 @@ def test_do_update_tuple_values(self):
             ],
         )
 
+    def test_do_update_no_target(self):
+        """
+        Make sure that `DO UPDATE` with no `target` raises an exception.
+        """
+        Band = self.Band
+
+        new_popularity = self.band.popularity + 1000
+
+        with self.assertRaises(ValueError) as manager:
+            Band.insert(
+                Band(name=self.band.name, popularity=new_popularity)
+            ).on_conflict(
+                action="DO UPDATE",
+                values=[(Band.popularity, new_popularity + 2000)],
+            ).run_sync()
+
+        self.assertEqual(
+            manager.exception.__str__(),
+            "The `target` option must be provided with DO UPDATE.",
+        )
+
     def test_do_update_no_values(self):
         """
         Make sure that `DO UPDATE` with no `values` raises an exception.