diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index 475f84e..83e6c72 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -16,9 +16,12 @@ class TagsController < ApplicationController
     q = params[:q].to_s.strip
     return render json: [] if q.blank?
 
+    with_nico = !(params[:nico].to_s.strip.downcase.in?(['0', 'false', 'off', 'no']))
+
     tags = (Tag.joins(:tag_name).includes(:tag_name)
-            .where('(tags.category = ? AND tag_names.name LIKE ?) OR tag_names.name LIKE ?',
-                   'nico', "nico:#{ q }%", "#{ q }%")
+            .where(((with_nico ? '(tags.category = ? AND tag_names.name LIKE ?) OR ' : '') +
+                    'tag_names.name LIKE ?'),
+                   *(with_nico ? ['nico', "nico:#{ q }%"] : []), "#{ q }%")
             .order(Arel.sql('post_count DESC, tag_names.name ASC'))
             .limit(20))
     render json: tags.as_json(only: [:id, :category, :post_count], methods: [:name, :has_wiki])
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index 1bd0723..18e873f 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -8,8 +8,10 @@ class Post < ApplicationRecord
   has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :post
   has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
   has_many :tags, through: :active_post_tags
+
   has_many :user_post_views, dependent: :destroy
-  has_many :post_similarities
+  has_many :post_similarities, dependent: :destroy
+
   has_one_attached :thumbnail
 
   before_validation :normalise_url
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 491a0e3..7952b58 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -3,53 +3,51 @@ class Tag < ApplicationRecord
     ;
   end
 
-  has_many :post_tags, dependent: :delete_all, inverse_of: :tag
+  has_many :post_tags, inverse_of: :tag
   has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :tag
   has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
   has_many :posts, through: :active_post_tags
-  has_many :tag_aliases, dependent: :destroy
 
   has_many :nico_tag_relations, foreign_key: :nico_tag_id, dependent: :destroy
   has_many :linked_tags, through: :nico_tag_relations, source: :tag
 
-  has_many :reversed_nico_tag_relations, class_name: 'NicoTagRelation',
-                                         foreign_key: :tag_id,
-                                         dependent: :destroy
+  has_many :reversed_nico_tag_relations,
+           class_name: 'NicoTagRelation', foreign_key: :tag_id, 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 :reversed_tag_implications,
+           class_name: 'TagImplication', foreign_key: :tag_id, dependent: :destroy
   has_many :parents, through: :reversed_tag_implications, source: :parent_tag
 
   belongs_to :tag_name
   delegate :name, to: :tag_name, allow_nil: true
   validates :tag_name, presence: true
 
-  enum :category, { deerjikist:  'deerjikist',
-                    meme:        'meme',
-                    character:   'character',
-                    general:     'general',
-                    material:    'material',
-                    nico:        'nico',
-                    meta:        'meta' }
+  enum :category, deerjikist:  'deerjikist',
+                  meme:        'meme',
+                  character:   'character',
+                  general:     'general',
+                  material:    'material',
+                  nico:        'nico',
+                  meta:        'meta'
 
   validates :category, presence: true, inclusion: { in: Tag.categories.keys }
 
   validate :nico_tag_name_must_start_with_nico
+  validate :tag_name_must_be_canonical
 
   scope :nico_tags, -> { where(category: :nico) }
 
   CATEGORY_PREFIXES = {
-      'gen:'  => 'general',
-      'djk:'  => 'deerjikist',
-      'meme:' => 'meme',
-      'chr:'  => 'character',
-      'mtr:'  => 'material',
-      'meta:' => 'meta' }.freeze
+      'gen:'  => :general,
+      'djk:'  => :deerjikist,
+      'meme:' => :meme,
+      'chr:'  => :character,
+      'mtr:'  => :material,
+      'meta:' => :meta }.freeze
 
   def name= val
     (self.tag_name ||= build_tag_name).name = val
@@ -60,11 +58,11 @@ class Tag < ApplicationRecord
   end
 
   def self.tagme
-    @tagme ||= find_or_create_by_tag_name!('タグ希望', category: 'meta')
+    @tagme ||= find_or_create_by_tag_name!('タグ希望', category: :meta)
   end
 
   def self.bot
-    @bot ||= find_or_create_by_tag_name!('bot操作', category: 'meta')
+    @bot ||= find_or_create_by_tag_name!('bot操作', category: :meta)
   end
 
   def self.normalise_tags tag_names, with_tagme: true, deny_nico: true
@@ -75,7 +73,7 @@ class Tag < ApplicationRecord
     tags = tag_names.map do |name|
       pf, cat = CATEGORY_PREFIXES.find { |p, _| name.start_with?(p) } || ['', nil]
       name = name.delete_prefix(pf)
-      find_or_create_by_tag_name!(name, category: (cat || 'general')).tap do |tag|
+      find_or_create_by_tag_name!(name, category: (cat || :general)).tap do |tag|
         if cat && tag.category != cat
           tag.update!(category: cat)
         end
@@ -127,4 +125,10 @@ class Tag < ApplicationRecord
       errors.add :name, 'ニコニコ・タグの命名規則に反してゐます．'
     end
   end
+
+  def tag_name_must_be_canonical
+    if tag_name&.canonical_id
+      errors.add :tag_name, 'tag_names へは実体を示す必要があります．'
+    end
+  end
 end
diff --git a/backend/app/models/tag_alias.rb b/backend/app/models/tag_alias.rb
deleted file mode 100644
index f695886..0000000
--- a/backend/app/models/tag_alias.rb
+++ /dev/null
@@ -1,6 +0,0 @@
-class TagAlias < ApplicationRecord
-  belongs_to :tag
-
-  validates :tag_id, presence: true
-  validates :name, presence: true, length: { maximum: 255 }, uniqueness: true
-end
diff --git a/backend/app/models/tag_name.rb b/backend/app/models/tag_name.rb
index 2efd022..9276e56 100644
--- a/backend/app/models/tag_name.rb
+++ b/backend/app/models/tag_name.rb
@@ -6,4 +6,21 @@ class TagName < ApplicationRecord
   has_many :aliases, class_name: 'TagName', foreign_key: :canonical_id
 
   validates :name, presence: true, length: { maximum: 255 }, uniqueness: true
+
+  validate :canonical_must_be_canonical
+  validate :alias_name_must_not_have_prefix
+
+  private
+
+  def canonical_must_be_canonical
+    if canonical&.canonical_id?
+      errors.add :canonical, 'canonical は実体を示す必要があります．'
+    end
+  end
+
+  def alias_name_must_not_have_prefix
+    if canonical_id? && name.to_s.include?(':')
+      errors.add :name, 'エーリアス名にプレフィクスを含むことはできません．'
+    end
+  end
 end