Fix for MagicParameters error with no POST body, closes #967

datasette/views/database.py

@@ -371,6 +371,11 @@ class MagicParameters(dict):
             )
         )
 
+    def __len__(self):
+        # Workaround for 'Incorrect number of bindings' error
+        # https://github.com/simonw/datasette/issues/967#issuecomment-692951144
+        return super().__len__() or 1
+
     def __getitem__(self, key):
         if key.startswith("_") and key.count("_") >= 2:
             prefix, suffix = key[1:].split("_", 1)

tests/test_canned_queries.py

@@ -305,14 +305,49 @@ def test_magic_parameters(magic_parameters_client, magic_parameter, expected_re)
     assert None is soup.find("input", {"name": magic_parameter})
     # Submit the form to create a log line
     response = magic_parameters_client.post(
-        "/data/runme_post", {}, csrftoken_from=True, cookies=cookies
+        "/data/runme_post?_json=1", {}, csrftoken_from=True, cookies=cookies
     )
+    assert response.json == {
+        "ok": True,
+        "message": "Query executed, 1 row affected",
+        "redirect": None,
+    }
     post_actual = magic_parameters_client.get(
         "/data/logs.json?_sort_desc=rowid&_shape=array"
     ).json[0]["line"]
     assert re.match(expected_re, post_actual)
 
 
+@pytest.mark.parametrize("use_csrf", [True, False])
+@pytest.mark.parametrize("return_json", [True, False])
+def test_magic_parameters_csrf_json(magic_parameters_client, use_csrf, return_json):
+    magic_parameters_client.ds._metadata["databases"]["data"]["queries"]["runme_post"][
+        "sql"
+    ] = "insert into logs (line) values (:_header_host)"
+    qs = ""
+    if return_json:
+        qs = "?_json=1"
+    response = magic_parameters_client.post(
+        "/data/runme_post{}".format(qs),
+        {},
+        csrftoken_from=use_csrf or None,
+        allow_redirects=False,
+    )
+    if return_json:
+        assert response.status == 200
+        assert response.json["ok"], response.json
+    else:
+        assert response.status == 302
+        messages = magic_parameters_client.ds.unsign(
+            response.cookies["ds_messages"], "messages"
+        )
+        assert [["Query executed, 1 row affected", 1]] == messages
+    post_actual = magic_parameters_client.get(
+        "/data/logs.json?_sort_desc=rowid&_shape=array"
+    ).json[0]["line"]
+    assert post_actual == "localhost"
+
+
 def test_magic_parameters_cannot_be_used_in_arbitrary_queries(magic_parameters_client):
     response = magic_parameters_client.get(
         "/data.json?sql=select+:_header_host&_shape=array"