From 2bf7ce5f517d772a16d7855a35a8a75d4456aad7 Mon Sep 17 00:00:00 2001
From: Simon Willison <swillison@gmail.com>
Date: Sat, 2 Nov 2019 16:12:46 -0700
Subject: [PATCH] Fix CSV export for nullable foreign keys, closes #612

---
 datasette/views/base.py | 12 ++++++++----
 tests/test_csv.py       | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/datasette/views/base.py b/datasette/views/base.py
index 1568b084..94945304 100644
--- a/datasette/views/base.py
+++ b/datasette/views/base.py
@@ -330,10 +330,14 @@ class DataView(BaseView):
                         else:
                             # Look for {"value": "label": } dicts and expand
                             new_row = []
-                            for cell in row:
-                                if isinstance(cell, dict):
-                                    new_row.append(cell["value"])
-                                    new_row.append(cell["label"])
+                            for heading, cell in zip(data["columns"], row):
+                                if heading in expanded_columns:
+                                    if cell is None:
+                                        new_row.extend(("", ""))
+                                    else:
+                                        assert isinstance(cell, dict)
+                                        new_row.append(cell["value"])
+                                        new_row.append(cell["label"])
                                 else:
                                     new_row.append(cell)
                             await writer.writerow(new_row)
diff --git a/tests/test_csv.py b/tests/test_csv.py
index b148b6db..13aca489 100644
--- a/tests/test_csv.py
+++ b/tests/test_csv.py
@@ -41,6 +41,14 @@ pk,created,planet_int,on_earth,state,city_id,city_id_label,neighborhood,tags,com
     "\n", "\r\n"
 )
 
+EXPECTED_TABLE_WITH_NULLABLE_LABELS_CSV = """
+pk,foreign_key_with_label,foreign_key_with_label_label,foreign_key_with_no_label,foreign_key_with_no_label_label
+1,1,hello,1,1
+2,,,,
+""".lstrip().replace(
+    "\n", "\r\n"
+)
+
 
 def test_table_csv(app_client):
     response = app_client.get("/fixtures/simple_primary_key.csv")
@@ -63,6 +71,13 @@ def test_table_csv_with_labels(app_client):
     assert EXPECTED_TABLE_WITH_LABELS_CSV == response.text
 
 
+def test_table_csv_with_nullable_labels(app_client):
+    response = app_client.get("/fixtures/foreign_key_references.csv?_labels=1")
+    assert response.status == 200
+    assert "text/plain; charset=utf-8" == response.headers["content-type"]
+    assert EXPECTED_TABLE_WITH_NULLABLE_LABELS_CSV == response.text
+
+
 def test_custom_sql_csv(app_client):
     response = app_client.get(
         "/fixtures.csv?sql=select+content+from+simple_primary_key+limit+2"