Don't remove notes when reply / renote is removed (#16287)

* chore: make NO ACTION on channel/reply/renote removal

* chore(docs): add description to show a possibility of reply null with replyId non-null

* fix: packing NoteDraft fails when reply / renote is removed

* feat: show drafts targeting removed renote / reply as "削除された投稿への投稿"

4 files changed, 49 insertions, 7 deletions

diff --git a/packages/backend/migration/1752502434151-no-action-on-draft-relation.js b/packages/backend/migration/1752502434151-no-action-on-draft-relation.js
new file mode 100644
index 0000000000..e3c63b79c7
--- /dev/null
+++ b/packages/backend/migration/1752502434151-no-action-on-draft-relation.js
@@ -0,0 +1,24 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export class NoActionOnDraftRelation1752502434151 {
+    name = 'NoActionOnDraftRelation1752502434151'
+
+    async up(queryRunner) {
+			await queryRunner.query(`ALTER TABLE "note_draft" DROP CONSTRAINT "FK_NOTE_DRAFT_CHANNEL_ID"`);
+			await queryRunner.query(`ALTER TABLE "note_draft" DROP CONSTRAINT "FK_NOTE_DRAFT_USER_ID"`);
+			await queryRunner.query(`ALTER TABLE "note_draft" DROP CONSTRAINT "FK_NOTE_DRAFT_RENOTE_ID"`);
+			await queryRunner.query(`ALTER TABLE "note_draft" DROP CONSTRAINT "FK_NOTE_DRAFT_REPLY_ID"`);
+			await queryRunner.query(`ALTER TABLE "note_draft" ADD CONSTRAINT "FK_e4983f28b4b18b03491536052f5" FOREIGN KEY ("userId") REFERENCES "user"("id") ON DELETE CASCADE ON UPDATE NO ACTION`);
+    }
+
+    async down(queryRunner) {
+			await queryRunner.query(`ALTER TABLE "note_draft" DROP CONSTRAINT "FK_e4983f28b4b18b03491536052f5"`);
+			await queryRunner.query(`ALTER TABLE "note_draft" ADD CONSTRAINT "FK_NOTE_DRAFT_REPLY_ID" FOREIGN KEY ("replyId") REFERENCES "note"("id") ON DELETE CASCADE ON UPDATE NO ACTION`);
+			await queryRunner.query(`ALTER TABLE "note_draft" ADD CONSTRAINT "FK_NOTE_DRAFT_RENOTE_ID" FOREIGN KEY ("renoteId") REFERENCES "note"("id") ON DELETE CASCADE ON UPDATE NO ACTION`);
+			await queryRunner.query(`ALTER TABLE "note_draft" ADD CONSTRAINT "FK_NOTE_DRAFT_USER_ID" FOREIGN KEY ("userId") REFERENCES "user"("id") ON DELETE CASCADE ON UPDATE NO ACTION`);
+			await queryRunner.query(`ALTER TABLE "note_draft" ADD CONSTRAINT "FK_NOTE_DRAFT_CHANNEL_ID" FOREIGN KEY ("channelId") REFERENCES "channel"("id") ON DELETE CASCADE ON UPDATE NO ACTION`);
+    }
+}
diff --git a/packages/backend/src/core/entities/NoteDraftEntityService.ts b/packages/backend/src/core/entities/NoteDraftEntityService.ts
index 26455029d5..3ef8cdaa12 100644
--- a/packages/backend/src/core/entities/NoteDraftEntityService.ts
+++ b/packages/backend/src/core/entities/NoteDraftEntityService.ts
@@ -5,6 +5,7 @@
 
 import { Inject, Injectable } from '@nestjs/common';
 import { ModuleRef } from '@nestjs/core';
+import { EntityNotFoundError } from 'typeorm';
 import { DI } from '@/di-symbols.js';
 import type { Packed } from '@/misc/json-schema.js';
 import { awaitAll } from '@/misc/prelude/await-all.js';
@@ -90,6 +91,17 @@ export class NoteDraftEntityService implements OnModuleInit {
 		const packedFiles = options?._hint_?.packedFiles;
 		const packedUsers = options?._hint_?.packedUsers;
 
+		async function nullIfEntityNotFound<T>(promise: Promise<T>): Promise<T | null> {
+			try {
+				return await promise;
+			} catch (err) {
+				if (err instanceof EntityNotFoundError) {
+					return null;
+				}
+				throw err;
+			}
+		}
+
 		const packed: Packed<'NoteDraft'> = await awaitAll({
 			id: noteDraft.id,
 			createdAt: this.idService.parse(noteDraft.id).date.toISOString(),
@@ -117,15 +129,15 @@ export class NoteDraftEntityService implements OnModuleInit {
 			} : undefined,
 
 			...(opts.detail ? {
-				reply: noteDraft.replyId ? this.noteEntityService.pack(noteDraft.replyId, me, {
+				reply: noteDraft.replyId ? nullIfEntityNotFound(this.noteEntityService.pack(noteDraft.replyId, me, {
 					detail: false,
 					skipHide: opts.skipHide,
-				}) : undefined,
+				})) : undefined,
 
-				renote: noteDraft.renoteId ? this.noteEntityService.pack(noteDraft.renoteId, me, {
+				renote: noteDraft.renoteId ? nullIfEntityNotFound(this.noteEntityService.pack(noteDraft.renoteId, me, {
 					detail: true,
 					skipHide: opts.skipHide,
-				}) : undefined,
+				})) : undefined,
 
 				poll: noteDraft.hasPoll ? {
 					choices: noteDraft.pollChoices,
diff --git a/packages/backend/src/models/NoteDraft.ts b/packages/backend/src/models/NoteDraft.ts
index edae254bb8..39d85e53dc 100644
--- a/packages/backend/src/models/NoteDraft.ts
+++ b/packages/backend/src/models/NoteDraft.ts
@@ -24,8 +24,9 @@ export class MiNoteDraft {
 	})
 	public replyId: MiNote['id'] | null;
 
+	// There is a possibility that replyId is not null but reply is null when the reply note is deleted.
 	@ManyToOne(type => MiNote, {
-		onDelete: 'CASCADE',
+		createForeignKeyConstraints: false,
 	})
 	@JoinColumn()
 	public reply: MiNote | null;
@@ -38,8 +39,9 @@ export class MiNoteDraft {
 	})
 	public renoteId: MiNote['id'] | null;
 
+	// There is a possibility that renoteId is not null but renote is null when the renote note is deleted.
 	@ManyToOne(type => MiNote, {
-		onDelete: 'CASCADE',
+		createForeignKeyConstraints: false,
 	})
 	@JoinColumn()
 	public renote: MiNote | null;
@@ -114,8 +116,10 @@ export class MiNoteDraft {
 	})
 	public channelId: MiChannel['id'] | null;
 
+	// There is a possibility that channelId is not null but channel is null when the channel is deleted.
+	// (deleting channel is not implemented so it's not happening now but may happen in the future)
 	@ManyToOne(type => MiChannel, {
-		onDelete: 'CASCADE',
+		createForeignKeyConstraints: false,
 	})
 	@JoinColumn()
 	public channel: MiChannel | null;
diff --git a/packages/backend/src/models/json-schema/note-draft.ts b/packages/backend/src/models/json-schema/note-draft.ts
index 20c56d0795..504b263a6d 100644
--- a/packages/backend/src/models/json-schema/note-draft.ts
+++ b/packages/backend/src/models/json-schema/note-draft.ts
@@ -51,11 +51,13 @@ export const packedNoteDraftSchema = {
 			type: 'object',
 			optional: true, nullable: true,
 			ref: 'Note',
+			description: 'The reply target note contents if exists. If the reply target has been deleted since the draft was created, this will be null while replyId is not null.',
 		},
 		renote: {
 			type: 'object',
 			optional: true, nullable: true,
 			ref: 'Note',
+			description: 'The renote target note contents if exists. If the renote target has been deleted since the draft was created, this will be null while renoteId is not null.',
 		},
 		visibility: {
 			type: 'string',