backend/app/controllers/posts_controller.rb

@@ -1,4 +1,6 @@
 class PostsController < ApplicationController
+  Event = Struct.new(:post, :tag, :user, :change_type, :timestamp, keyword_init: true)
+
   # GET /posts
   def index
     limit = params[:limit].presence&.to_i
@@ -128,6 +130,44 @@ class PostsController < ApplicationController
   def destroy
   end
 
+  def changes
+    id = params[:id]
+    page = (params[:page].presence || 1).to_i
+    limit = (params[:limit].presence || 20).to_i
+
+    page = 1 if page < 1
+    limit = 1 if limit < 1
+
+    offset = (page - 1) * limit
+
+    pts = PostTag.with_discarded
+    pts = pts.where(post_id: id) if id.present?
+    pts = pts.includes(:post, :tag, :created_user, :deleted_user)
+
+    events = []
+    pts.each do |pt|
+      events << Event.new(
+          post:         pt.post,
+          tag:          pt.tag,
+          user:         pt.created_user && { id: pt.created_user.id, name: pt.created_user.name },
+          change_type:  'add',
+          timestamp:    pt.created_at)
+
+      if pt.discarded_at
+        events << Event.new(
+            post:         pt.post,
+            tag:          pt.tag,
+            user:         pt.deleted_user && { id: pt.deleted_user.id, name: pt.deleted_user.name },
+            change_type:  'remove',
+            timestamp:    pt.discarded_at)
+      end
+    end
+    events.sort_by!(&:timestamp)
+    events.reverse!
+
+    render json: { changes: events.slice(offset, limit).as_json, count: events.size }
+  end
+
   private
 
   def filtered_posts

backend/config/routes.rb

@@ -4,6 +4,7 @@ Rails.application.routes.draw do
   get 'tags/autocomplete', to: 'tags#autocomplete'
   get 'tags/name/:name', to: 'tags#show_by_name'
   get 'posts/random', to: 'posts#random'
+  get 'posts/changes', to: 'posts#changes'
   post 'posts/:id/viewed', to: 'posts#viewed'
   delete 'posts/:id/viewed', to: 'posts#unviewed'
   get 'preview/title', to: 'preview#title'

frontend/src/App.tsx

@@ -9,6 +9,7 @@ import { API_BASE_URL } from '@/config'
 import NicoTagListPage from '@/pages/tags/NicoTagListPage'
 import NotFound from '@/pages/NotFound'
 import PostDetailPage from '@/pages/posts/PostDetailPage'
+import PostHistoryPage from '@/pages/posts/PostHistoryPage'
 import PostListPage from '@/pages/posts/PostListPage'
 import PostNewPage from '@/pages/posts/PostNewPage'
 import ServiceUnavailable from '@/pages/ServiceUnavailable'
@@ -79,6 +80,7 @@ export default (() => {
 	    <Route path="/posts" element={<PostListPage/>}/>
 	    <Route path="/posts/new" element={<PostNewPage user={user}/>}/>
 	    <Route path="/posts/:id" element={<PostDetailPage user={user}/>}/>
+	    <Route path="/posts/changes" element={<PostHistoryPage/>}/>
 	    <Route path="/tags/nico" element={<NicoTagListPage user={user}/>}/>
 	    <Route path="/wiki" element={<WikiSearchPage/>}/>
 	    <Route path="/wiki/:title" element={<WikiDetailPage/>}/>

frontend/src/components/TopNav.tsx

@@ -30,6 +30,7 @@ export default (({ user }: Props) => {
       { name: '広場', to: '/posts', subMenu: [
 	  { name: '一覧', to: '/posts' },
 	  { name: '投稿追加', to: '/posts/new' },
+	  { name: '耕作履歴', to: '/posts/changes' },
 	  { name: 'ヘルプ', to: '/wiki/ヘルプ:広場' }] },
       { name: 'タグ', to: '/tags', subMenu: [
 	  { name: 'タグ一覧', to: '/tags', visible: false },

frontend/src/components/common/Pagination.tsx

@@ -0,0 +1,79 @@
+import { Link, useLocation } from 'react-router-dom'
+
+import type { FC } from 'react'
+
+type Props = { page:          number
+	       totalPages:    number
+	       siblingCount?: number }
+
+
+const range = (start: number, end: number): number[] =>
+    [...Array (end - start + 1).keys ()].map (i => start + i)
+
+
+const getPages = (
+    page:         number,
+    total:        number,
+    siblingCount: number,
+): (number | '…')[] => {
+  if (total <= 1)
+    return [1]
+
+  const first = 1
+  const last = total
+
+  const left = Math.max (page - siblingCount, first)
+  const right = Math.min (page + siblingCount, last)
+
+  const pages: (number | '…')[] = []
+
+  pages.push (first)
+
+  if (left > first + 1)
+    pages.push ('…')
+
+  const midStart = Math.max (left, first + 1)
+  const midEnd = Math.min (right, last - 1)
+  pages.push (...range (midStart, midEnd))
+
+  if (right < last - 1)
+    pages.push ('…')
+
+  if (last !== first)
+    pages.push (last)
+
+  return pages.filter ((v, i, arr) => i === 0 || v !== arr[i - 1])
+}
+
+
+export default (({ page, totalPages, siblingCount = 4 }) => {
+  const location = useLocation ()
+
+  const buildTo = (p: number) => {
+    const qs = new URLSearchParams (location.search)
+    qs.set ('page', String (p))
+    return `${ location.pathname }?${ qs.toString () }`
+  }
+
+  const pages = getPages (page, totalPages, siblingCount)
+
+  return (
+      <nav className="mt-4 flex justify-center" aria-label="Pagination">
+	<div className="flex items-center gap-2">
+	  {(page > 1)
+	   ? <Link to={buildTo (page - 1)} aria-label="前のページ">&lt;</Link>
+	   : <span aria-hidden>&lt;</span>}
+
+	  {pages.map ((p, idx) => (
+	      (p === '…')
+	      ? <span key={`dots-${ idx }`}>…</span>
+	      : ((p === page)
+		 ? <span key={p} className="font-bold" aria-current="page">{p}</span>
+		 : <Link key={p} to={buildTo (p)}>{p}</Link>)))}
+
+	  {(page < totalPages)
+	   ? <Link to={buildTo (page + 1)} aria-label="次のページ">&gt;</Link>
+	   : <span aria-hidden>&gt;</span>}
+	</div>
+      </nav>)
+}) satisfies FC<Props>

frontend/src/pages/posts/PostHistoryPage.tsx

@@ -0,0 +1,90 @@
+import axios from 'axios'
+import toCamel from 'camelcase-keys'
+import { useEffect, useState } from 'react'
+import { Helmet } from 'react-helmet-async'
+import { Link, useLocation } from 'react-router-dom'
+
+import TagLink from '@/components/TagLink'
+import PageTitle from '@/components/common/PageTitle'
+import Pagination from '@/components/common/Pagination'
+import MainArea from '@/components/layout/MainArea'
+import { API_BASE_URL, SITE_TITLE } from '@/config'
+
+import type { FC } from 'react'
+
+import type { PostTagChange } from '@/types'
+
+
+export default (() => {
+  const [changes, setChanges] = useState<PostTagChange[]> ([])
+  const [totalPages, setTotalPages] = useState<number> (0)
+
+  const location = useLocation ()
+  const query = new URLSearchParams (location.search)
+  const id = query.get ('id')
+  const page = Number (query.get ('page') ?? 1)
+  const limit = Number (query.get ('limit') ?? 20)
+
+  useEffect (() => {
+    void (async () => {
+      const res = await axios.get (`${ API_BASE_URL }/posts/changes`,
+				   { params: { ...(id && { id }),
+					       ...(page && { page }),
+					       ...(limit && { limit }) } })
+      const data = toCamel (res.data as any, { deep: true }) as {
+	  changes:  PostTagChange[]
+	  count:    number }
+      setChanges (data.changes)
+      setTotalPages (Math.trunc ((data.count - 1) / limit))
+    }) ()
+  }, [location.search])
+
+  return (
+      <MainArea>
+	<Helmet>
+	  <title>{`耕作履歴 | ${ SITE_TITLE }`}</title>
+	</Helmet>
+
+	<PageTitle>
+	  耕作履歴
+	  {Boolean (id) && <>: 投稿 {<Link to={`/posts/${ id }`}>#{id}</Link>}</>}
+	</PageTitle>
+
+	<table className="table-auto w-full border-collapse">
+	  <thead>
+	    <tr>
+	      <th className="p-2 text-left">投稿</th>
+	      <th className="p-2 text-left">変更</th>
+	      <th className="p-2 text-left">日時</th>
+	    </tr>
+	  </thead>
+	  <tbody>
+	    {changes.map (change => (
+		<tr key={`${ change.timestamp }-${ change.post.id }-${ change.tag.id }`}>
+		  <td>
+		    <Link to={`/posts/${ change.post.id }`}>
+		      <img src={change.post.thumbnail || change.post.thumbnailBase || undefined}
+			   alt={change.post.title || change.post.url}
+			   title={change.post.title || change.post.url || undefined}
+			   className="w-40"/>
+		    </Link>
+		  </td>
+		  <td>
+		    <TagLink tag={change.tag} withWiki={false} withCount={false}/>
+		    {`を${ change.changeType === 'add' ? '追加' : '削除' }`}
+		  </td>
+		  <td>
+		    {change.user ? (
+			<Link to={`/users/${ change.user.id }`}>
+			  {change.user.name}
+			</Link>) : 'bot 操作'}
+		    <br/>
+		    {change.timestamp}
+		  </td>
+		</tr>))}
+	  </tbody>
+	</table>
+
+	<Pagination page={page} totalPages={totalPages}/>
+      </MainArea>)
+}) satisfies FC

frontend/src/types.ts

@@ -29,6 +29,13 @@ export type Post = {
     originalCreatedFrom:    string | null
     originalCreatedBefore:  string | null }
 
+export type PostTagChange = {
+    post:       Post
+    tag:        Tag
+    user?:      User
+    changeType: 'add' | 'remove'
+    timestamp:  string }
+
 export type SubMenuItem =
     | { component:  ReactNode
 	visible:    boolean }