Ensure django database wrappers always have a connection before creating a cursor

Only used during ocrpus export. The .using() calls work properly.

I guess the low-level code expect that DB to be used at least once before calling the .cursor()

arkindex/documents/export/__init__.py

@@ -51,7 +51,13 @@ def run_pg_query(query, source_db):
     Run a single Postgresql query and split the results into chunks.
     When a name is given to a cursor, psycopg2 uses a server-side cursor; we just use a random string as a name.
     """
-    with connections[source_db].create_cursor(name=str(uuid.uuid4())) as pg_cursor:
+    db = connections[source_db]
+
+    # Make sure a connection is open and available for export databases
+    if source_db != "default" and db.connection is None:
+        db.connect()
+
+    with db.create_cursor(name=str(uuid.uuid4())) as pg_cursor:
         pg_cursor.itersize = BATCH_SIZE
         pg_cursor.execute(query)