Try to find annotation before importing as a new one

We sort annotations that we want to import in descending `updated` order.
Which means that when we import annotation X on the first run, and this
annotation gets updated before the second run, we will have to try and
find annotations, and on success update annotation X instead of creating
a new one which would lead to a unique constraint error on the `id`
column.

This use case needs to be handled as we want to run the migration
multiple times to ensure we have all the new and updated annotations
correctly migrated to Postgres.

scripts/migrate.py

@@ -183,7 +183,10 @@ def annotation_from_data(id, data):
     if id == '_query':
         raise Skip("not an annotation (id=_query)")
 
-    ann = Annotation(id=id)
+    ann = Annotation.query.get(id)
+    if ann is None:
+        ann = Annotation(id=id)
+
     ann.created = dateparser.parse(data.pop('created')).replace(tzinfo=None)
     ann.updated = dateparser.parse(data.pop('updated')).replace(tzinfo=None)
     ann.userid = data.pop('user')