[BUG FIX] Handle quasi dupes during migration [MER-2954] (#4604)

* adjust migration to be robust against bad data

* Auto format

---------

Co-authored-by: darrensiegel <darrensiegel@users.noreply.github.com>

priv/repo/migrations/20240125204755_fix_revision_parts.exs

@@ -2,12 +2,52 @@ defmodule Oli.Repo.Migrations.FixRevisionParts do
   use Ecto.Migration
 
   def change do
+    # Extract the unique entries in a new table.  This will take records
+    # like this:
+    #
+    # revision_id | part_id | grading_approach
+    # 1           | 1       | automatic
+    # 1           | 1       | NULL
+    # 1           | 1       | NULL
+    # 1           | 2       | NULL
+    # 1           | 2       | NULL
+    # 1           | 2       | NULL
+    # 1           | 2       | NULL
+    # 1           | 2       | NULL
+    # 1           | 2       | NULL
+    # 1           | 2       | NULL
+    #
+    # To:
+    # revision_id | part_id | grading_approach
+    # 1           | 1       | automatic
+    # 1           | 1       | NULL
+    # 1           | 2       | NULL
     execute """
     CREATE TABLE temp_revision_parts AS
     SELECT DISTINCT ON (revision_id, grading_approach, part_id) *
     FROM revision_parts;
     """
 
+    # Delete the duplicate entries that have a NULL grading_approach
+    # This will take records like this and delete the record with the NULL:
+    #
+    # revision_id | part_id | grading_approach
+    # 1           | 1       | automatic
+    # 1           | 1       | NULL
+    execute """
+    DELETE FROM temp_revision_parts
+    WHERE (revision_id, part_id) IN (
+        SELECT revision_id, part_id
+        FROM temp_revision_parts
+        GROUP BY revision_id, part_id
+        HAVING COUNT(*) > 1
+    )
+    AND grading_approach IS NULL;
+    """
+
+    # At this point we have a correctly structured copy of revision_parts, so
+    # we can backup the origin, drop its indices and rename the new table to
+    # to be revision_parts:
     execute """
     ALTER TABLE revision_parts RENAME TO backup_revision_parts;
     """
@@ -30,6 +70,7 @@ defmodule Oli.Repo.Migrations.FixRevisionParts do
 
     flush()
 
+    # Finally, add back in the indices on the new revision_parts table:
     create unique_index(:revision_parts, [:revision_id, :part_id, :grading_approach])
     create index(:revision_parts, [:revision_id])
   end