[themotte/rDrama#451] Backfill comments.descendant_count column

migrations/versions/2023_01_03_07_59_56_1f30a37b08a0_dml_populate_comments_descendant_count_.py

@@ -0,0 +1,45 @@
+"""DML: populate comments.descendant_count column
+
+Revision ID: 1f30a37b08a0
+Revises: f8ba0e88ddd1
+Create Date: 2023-01-03 07:59:56.599209+00:00
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.sql.expression import func
+
+from files.__main__ import db_session
+from files.classes import Comment
+
+# revision identifiers, used by Alembic.
+revision = '1f30a37b08a0'
+down_revision = 'f8ba0e88ddd1'
+branch_labels = None
+depends_on = None
+
+def upgrade():
+	db = db_session()
+
+	max_level = db.query(func.max(Comment.level)).scalar()
+	for level in range(max_level, 0, -1):
+		db.execute('''
+			WITH "descendant_counts" as (
+				SELECT
+					"parent_comments"."id",
+					COALESCE(SUM(1 + "child_comments"."descendant_count"), 0) as "descendant_count"
+				FROM "comments" as "parent_comments"
+					LEFT JOIN "comments" AS "child_comments" ON "child_comments"."parent_comment_id" = "parent_comments"."id"
+				GROUP BY "parent_comments"."id"
+			)
+			UPDATE "comments"
+			SET "descendant_count" = "descendant_counts"."descendant_count"
+			FROM "descendant_counts"
+			WHERE "comments"."id" = "descendant_counts"."id"
+		''', {"level": level})
+	db.commit()
+
+
+def downgrade():
+	db = db_session()
+	db.query(Comment).update({'descendant_count': 0})