backend/app/controllers/posts_controller.rb

@@ -52,9 +52,12 @@ class PostsController < ApplicationController
 
     viewed = current_user&.viewed?(post) || false
 
-    render json: (post
-                  .as_json(include: { tags: { only: [:id, :name, :category, :post_count] } })
-                  .merge(related: post.related(limit: 20), viewed:))
+    json = post.as_json
+    json['tags'] = build_tag_tree_for(post.tags)
+    json['related'] = post.related(limit: 20)
+    json['viewed'] = viewed
+
+    render json:
   end
 
   # POST /posts
@@ -78,6 +81,7 @@ class PostsController < ApplicationController
     if post.save
       post.resized_thumbnail!
       post.tags = Tag.normalise_tags(tag_names)
+      post.tags = Tag.expand_parent_tags(post.tags)
       render json: post.as_json(include: { tags: { only: [:id, :name, :category, :post_count] } }),
              status: :created
     else
@@ -112,9 +116,11 @@ class PostsController < ApplicationController
     post = Post.find(params[:id].to_i)
     tags = post.tags.where(category: 'nico').to_a +
            Tag.normalise_tags(tag_names, with_tagme: false)
+    tags = Tag.expand_parent_tags(tags)
     if post.update(title:, tags:, original_created_from:, original_created_before:)
-      render json: post.as_json(include: { tags: { only: [:id, :name, :category, :post_count] } }),
-             status: :ok
+      json = post.as_json
+      json['tags'] = build_tag_tree_for(post.tags)
+      render json:, status: :ok
     else
       render json: post.errors, status: :unprocessable_entity
     end
@@ -143,4 +149,46 @@ class PostsController < ApplicationController
     end
     posts.distinct
   end
+
+  def build_tag_tree_for tags
+    tags = tags.to_a
+    tag_ids = tags.map(&:id)
+
+    implications = TagImplication.where(parent_tag_id: tag_ids, tag_id: tag_ids)
+
+    children_ids_by_parent = Hash.new { |h, k| h[k] = [] }
+    implications.each do |imp|
+      children_ids_by_parent[imp.parent_tag_id] << imp.tag_id
+    end
+
+    child_ids = children_ids_by_parent.values.flatten.uniq
+
+    root_ids = tag_ids - child_ids
+
+    tags_by_id = tags.index_by(&:id)
+
+    memo = { }
+
+    build_node = -> tag_id, path do
+      tag = tags_by_id[tag_id]
+      return nil unless tag
+
+      if path.include?(tag_id)
+        return tag.as_json(only: [:id, :name, :category, :post_count]).merge(children: [])
+      end
+
+      if memo.key?(tag_id)
+        return memo[tag_id]
+      end
+
+      new_path = path + [tag_id]
+      child_ids = children_ids_by_parent[tag_id] || []
+
+      children = child_ids.filter_map { |cid| build_node.(cid, new_path) }
+
+      memo[tag_id] = tag.as_json(only: [:id, :name, :category, :post_count]).merge(children:)
+    end
+
+    root_ids.filter_map { |id| build_node.call(id, []) }
+  end
 end

backend/app/models/tag.rb

@@ -11,6 +11,14 @@ class Tag < ApplicationRecord
                                          dependent: :destroy
   has_many :linked_nico_tags, through: :reversed_nico_tag_relations, source: :nico_tag
 
+  has_many :tag_implications, foreign_key: :parent_tag_id, dependent: :destroy
+  has_many :children, through: :tag_implications, source: :tag
+
+  has_many :reversed_tag_implications, class_name: 'TagImplication',
+                                       foreign_key: :tag_id,
+                                       dependent: :destroy
+  has_many :parents, through: :reversed_tag_implications, source: :parent_tag
+
   enum :category, { deerjikist:  'deerjikist',
                     meme:        'meme',
                     character:   'character',
@@ -61,6 +69,29 @@ class Tag < ApplicationRecord
     tags.uniq
   end
 
+  def self.expand_parent_tags tags
+    return [] if tags.blank?
+
+    seen = Set.new
+    result = []
+    stack = tags.compact.dup
+
+    until stack.empty?
+      tag = stack.pop
+      next unless tag
+
+      tag.parents.each do |parent|
+        next if seen.include?(parent.id)
+
+        seen << parent.id
+        result << parent
+        stack << parent
+      end
+    end
+
+    (result + tags).uniq { |t| t.id }
+  end
+
   private
 
   def nico_tag_name_must_start_with_nico

backend/app/models/tag_implication.rb

@@ -0,0 +1,17 @@
+class TagImplication < ApplicationRecord
+  belongs_to :tag, class_name: 'Tag'
+  belongs_to :parent_tag, class_name: 'Tag'
+
+  validates :tag_id, presence: true
+  validates :parent_tag_id, presence: true
+
+  validate :parent_tag_mustnt_be_itself
+
+  private
+
+  def parent_tag_mustnt_be_itself
+    if parent_tag == tag
+      errors.add :parent_tag_id, '親タグは子タグと同一であってはなりません．'
+    end
+  end
+end

backend/db/migrate/20251009222200_create_tag_implications.rb

@@ -0,0 +1,9 @@
+class CreateTagImplications < ActiveRecord::Migration[8.0]
+  def change
+    create_table :tag_implications do |t|
+      t.references :tag, null: false, foreign_key: { to_table: :tags }
+      t.references :parent_tag, null: false, foreign_key: { to_table: :tags }
+      t.timestamps
+    end
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2025_11_26_231500) do
+ActiveRecord::Schema[8.0].define(version: 2025_10_09_222200) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false

frontend/src/components/PostEditForm.tsx

@@ -1,6 +1,6 @@
 import axios from 'axios'
 import toCamel from 'camelcase-keys'
-import { useState } from 'react'
+import { useEffect, useState } from 'react'
 
 import PostFormTagsArea from '@/components/PostFormTagsArea'
 import PostOriginalCreatedTimeField from '@/components/PostOriginalCreatedTimeField'
@@ -10,7 +10,23 @@ import { API_BASE_URL } from '@/config'
 
 import type { FC } from 'react'
 
-import type { Post } from '@/types'
+import type { Post, Tag } from '@/types'
+
+
+const tagsToStr = (tags: Tag[]): string => {
+  const result: Tag[] = []
+
+  const walk = (tag: Tag) => {
+    const { children, ...rest } = tag
+    result.push (rest)
+    children?.forEach (walk)
+  }
+
+  tags.filter (t => t.category !== 'nico').forEach (walk)
+
+  return [...(new Set (result.map (t => t.name)))].join (' ')
+}
+
 
 type Props = { post:    Post
 	       onSave:  (newPost: Post) => void }
@@ -22,10 +38,7 @@ export default (({ post, onSave }: Props) => {
   const [originalCreatedFrom, setOriginalCreatedFrom] =
       useState<string | null> (post.originalCreatedFrom)
   const [title, setTitle] = useState (post.title)
-  const [tags, setTags] = useState<string> (post.tags
-					    .filter (t => t.category !== 'nico')
-					    .map (t => t.name)
-					    .join (' '))
+  const [tags, setTags] = useState<string> ('')
 
   const handleSubmit = async () => {
     const res = await axios.put (
@@ -43,6 +56,10 @@ export default (({ post, onSave }: Props) => {
 	      originalCreatedBefore:  data.originalCreatedBefore } as Post)
   }
 
+  useEffect (() => {
+    setTags(tagsToStr (post.tags))
+  }, [post])
+
   return (
       <div className="max-w-xl pt-2 space-y-4">
 	{/* タイトル */}

frontend/src/components/TagDetailSidebar.tsx

@@ -7,12 +7,30 @@ import SubsectionTitle from '@/components/common/SubsectionTitle'
 import SidebarComponent from '@/components/layout/SidebarComponent'
 import { CATEGORIES } from '@/consts'
 
-import type { FC } from 'react'
+import type { FC, ReactNode } from 'react'
 
 import type { Category, Post, Tag } from '@/types'
 
 type TagByCategory = { [key in Category]: Tag[] }
 
+
+const renderTagTree = (
+    tag:        Tag,
+    nestLevel:  number,
+    path:       string,
+    ): ReactNode[] => {
+  const key = `${ path }-${ tag.id }`
+
+  const self = (
+      <li key={key} className="mb-1">
+	<TagLink tag={tag} nestLevel={nestLevel}/>
+      </li>)
+
+  return [self,
+	  ...(tag.children?.flatMap (child => renderTagTree (child, nestLevel + 1, key)) ?? [])]
+}
+
+
 type Props = { post: Post | null }
 
 
@@ -54,10 +72,7 @@ export default (({ post }: Props) => {
 	    <div className="my-3" key={cat}>
 	      <SubsectionTitle>{categoryNames[cat]}</SubsectionTitle>
 	      <ul>
-		{tags[cat].map (tag => (
-		    <li key={tag.id} className="mb-1">
-		      <TagLink tag={tag}/>
-		    </li>))}
+		{tags[cat].map (tag => renderTagTree (tag, 0, `cat-${ cat }`))}
 	      </ul>
 	    </div>))}
 	{post && (

frontend/src/components/TagLink.tsx

@@ -8,6 +8,7 @@ import type { ComponentProps, FC, HTMLAttributes } from 'react'
 import type { Tag } from '@/types'
 
 type CommonProps = { tag:         Tag
+		     nestLevel?:  number
 		     withWiki?:   boolean
 		     withCount?:  boolean }
 
@@ -21,6 +22,7 @@ type Props = PropsWithLink | PropsWithoutLink
 
 
 export default (({ tag,
+		   nestLevel = 0,
 		   linkFlg   = true,
 		   withWiki  = true,
 		   withCount = true,
@@ -42,6 +44,12 @@ export default (({ tag,
 		?
 	      </Link>
 	    </span>)}
+	{nestLevel > 0 && (
+	    <span
+		className="ml-1 mr-1"
+		style={{ paddingLeft: `${ (nestLevel - 1) }rem` }}>
+	      ↳
+	    </span>)}
 	{linkFlg
 	 ? (
 	    <Link to={`/posts?${ (new URLSearchParams ({ tags: tag.name })).toString () }`}

frontend/src/types.ts

@@ -1,7 +1,7 @@
-import React from 'react'
-
 import { CATEGORIES, USER_ROLES, ViewFlagBehavior } from '@/consts'
 
+import type { ReactNode } from 'react'
+
 export type Category = typeof CATEGORIES[number]
 
 export type Menu = MenuItem[]
@@ -29,19 +29,19 @@ export type Post = {
     originalCreatedFrom:    string | null
     originalCreatedBefore:  string | null }
 
-export type SubMenuItem = {
-    component:  React.ReactNode
-    visible:    boolean
-    } | {
-    name:     string
-    to:       string
-    visible?: boolean }
+export type SubMenuItem =
+    | { component:  ReactNode
+	visible:    boolean }
+    | { name:     string
+	to:       string
+	visible?: boolean }
 
 export type Tag = {
     id:         number
     name:       string
     category:   Category
-    postCount:  number }
+    postCount:  number
+    children?:  Tag[] }
 
 export type User = {
     id:               number