From c3f0621a59a74d0e20e6db6170894871c48e8f0f Mon Sep 17 00:00:00 2001
From: Eugen Rochko <eugen@zeonfederated.com>
Date: Sun, 17 Jul 2022 13:49:29 +0200
Subject: [PATCH] Add ability to follow hashtags (#18809)

---
 .../featured_tags/suggestions_controller.rb   |  2 +-
 .../api/v1/followed_tags_controller.rb        | 52 ++++++++++++
 app/controllers/api/v1/tags_controller.rb     | 29 +++++++
 .../api/v1/trends/tags_controller.rb          |  2 +-
 app/lib/feed_manager.rb                       | 36 +++++---
 app/models/tag.rb                             |  5 +-
 app/models/tag_follow.rb                      | 24 ++++++
 app/presenters/tag_relationships_presenter.rb | 15 ++++
 app/serializers/rest/tag_serializer.rb        | 14 ++++
 app/services/fan_out_on_write_service.rb      | 15 +++-
 app/workers/feed_insert_worker.rb             |  8 +-
 config/routes.rb                              |  9 ++
 .../20220714171049_create_tag_follows.rb      | 12 +++
 db/schema.rb                                  | 13 ++-
 .../api/v1/followed_tags_controller_spec.rb   | 23 ++++++
 .../api/v1/tags_controller_spec.rb            | 82 +++++++++++++++++++
 spec/fabricators/tag_follow_fabricator.rb     |  4 +
 spec/models/tag_follow_spec.rb                |  4 +
 18 files changed, 329 insertions(+), 20 deletions(-)
 create mode 100644 app/controllers/api/v1/followed_tags_controller.rb
 create mode 100644 app/controllers/api/v1/tags_controller.rb
 create mode 100644 app/models/tag_follow.rb
 create mode 100644 app/presenters/tag_relationships_presenter.rb
 create mode 100644 db/migrate/20220714171049_create_tag_follows.rb
 create mode 100644 spec/controllers/api/v1/followed_tags_controller_spec.rb
 create mode 100644 spec/controllers/api/v1/tags_controller_spec.rb
 create mode 100644 spec/fabricators/tag_follow_fabricator.rb
 create mode 100644 spec/models/tag_follow_spec.rb

diff --git a/app/controllers/api/v1/featured_tags/suggestions_controller.rb b/app/controllers/api/v1/featured_tags/suggestions_controller.rb
index 75545d3c7..76633210a 100644
--- a/app/controllers/api/v1/featured_tags/suggestions_controller.rb
+++ b/app/controllers/api/v1/featured_tags/suggestions_controller.rb
@@ -6,7 +6,7 @@ class Api::V1::FeaturedTags::SuggestionsController < Api::BaseController
   before_action :set_recently_used_tags, only: :index
 
   def index
-    render json: @recently_used_tags, each_serializer: REST::TagSerializer
+    render json: @recently_used_tags, each_serializer: REST::TagSerializer, relationships: TagRelationshipsPresenter.new(@recently_used_tags, current_user&.account_id)
   end
 
   private
diff --git a/app/controllers/api/v1/followed_tags_controller.rb b/app/controllers/api/v1/followed_tags_controller.rb
new file mode 100644
index 000000000..f0dfd044c
--- /dev/null
+++ b/app/controllers/api/v1/followed_tags_controller.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+class Api::V1::FollowedTagsController < Api::BaseController
+  TAGS_LIMIT = 100
+
+  before_action -> { doorkeeper_authorize! :follow, :read, :'read:follows' }, except: :show
+  before_action :require_user!
+  before_action :set_results
+
+  after_action :insert_pagination_headers, only: :show
+
+  def index
+    render json: @results.map(&:tag), each_serializer: REST::TagSerializer, relationships: TagRelationshipsPresenter.new(@results.map(&:tag), current_user&.account_id)
+  end
+
+  private
+
+  def set_results
+    @results = TagFollow.where(account: current_account).joins(:tag).eager_load(:tag).to_a_paginated_by_id(
+      limit_param(TAGS_LIMIT),
+      params_slice(:max_id, :since_id, :min_id)
+    )
+  end
+
+  def insert_pagination_headers
+    set_pagination_headers(next_path, prev_path)
+  end
+
+  def next_path
+    api_v1_followed_tags_url pagination_params(max_id: pagination_max_id) if records_continue?
+  end
+
+  def prev_path
+    api_v1_followed_tags_url pagination_params(since_id: pagination_since_id) unless @results.empty?
+  end
+
+  def pagination_max_id
+    @results.last.id
+  end
+
+  def pagination_since_id
+    @results.first.id
+  end
+
+  def records_continue?
+    @results.size == limit_param(TAG_LIMIT)
+  end
+
+  def pagination_params(core_params)
+    params.slice(:limit).permit(:limit).merge(core_params)
+  end
+end
diff --git a/app/controllers/api/v1/tags_controller.rb b/app/controllers/api/v1/tags_controller.rb
new file mode 100644
index 000000000..d45015ff5
--- /dev/null
+++ b/app/controllers/api/v1/tags_controller.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class Api::V1::TagsController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :follow, :write, :'write:follows' }, except: :show
+  before_action :require_user!, except: :show
+  before_action :set_or_create_tag
+
+  override_rate_limit_headers :follow, family: :follows
+
+  def show
+    render json: @tag, serializer: REST::TagSerializer
+  end
+
+  def follow
+    TagFollow.create!(tag: @tag, account: current_account, rate_limit: true)
+    render json: @tag, serializer: REST::TagSerializer
+  end
+
+  def unfollow
+    TagFollow.find_by(account: current_account, tag: @tag)&.destroy!
+    render json: @tag, serializer: REST::TagSerializer
+  end
+
+  private
+
+  def set_or_create_tag
+    @tag = Tag.find_normalized(params[:id]) || Tag.new(name: Tag.normalize(params[:id]), display_name: params[:id])
+  end
+end
diff --git a/app/controllers/api/v1/trends/tags_controller.rb b/app/controllers/api/v1/trends/tags_controller.rb
index 41f9ffac1..21adfa2a1 100644
--- a/app/controllers/api/v1/trends/tags_controller.rb
+++ b/app/controllers/api/v1/trends/tags_controller.rb
@@ -8,7 +8,7 @@ class Api::V1::Trends::TagsController < Api::BaseController
   DEFAULT_TAGS_LIMIT = 10
 
   def index
-    render json: @tags, each_serializer: REST::TagSerializer
+    render json: @tags, each_serializer: REST::TagSerializer, relationships: TagRelationshipsPresenter.new(@tags, current_user&.account_id)
   end
 
   private
diff --git a/app/lib/feed_manager.rb b/app/lib/feed_manager.rb
index 2eb4ba2f4..145352fe8 100644
--- a/app/lib/feed_manager.rb
+++ b/app/lib/feed_manager.rb
@@ -45,6 +45,8 @@ class FeedManager
       filter_from_list?(status, receiver) || filter_from_home?(status, receiver.account_id, build_crutches(receiver.account_id, [status]))
     when :mentions
       filter_from_mentions?(status, receiver.id)
+    when :tags
+      filter_from_tags?(status, receiver.id, build_crutches(receiver.id, [status]))
     else
       false
     end
@@ -56,7 +58,7 @@ class FeedManager
   # @param [Boolean] update
   # @return [Boolean]
   def push_to_home(account, status, update: false)
-    return false unless add_to_feed(:home, account.id, status, account.user&.aggregates_reblogs?)
+    return false unless add_to_feed(:home, account.id, status, aggregate_reblogs: account.user&.aggregates_reblogs?)
 
     trim(:home, account.id)
     PushUpdateWorker.perform_async(account.id, status.id, "timeline:#{account.id}", { 'update' => update }) if push_update_required?("timeline:#{account.id}")
@@ -69,7 +71,7 @@ class FeedManager
   # @param [Boolean] update
   # @return [Boolean]
   def unpush_from_home(account, status, update: false)
-    return false unless remove_from_feed(:home, account.id, status, account.user&.aggregates_reblogs?)
+    return false unless remove_from_feed(:home, account.id, status, aggregate_reblogs: account.user&.aggregates_reblogs?)
 
     redis.publish("timeline:#{account.id}", Oj.dump(event: :delete, payload: status.id.to_s)) unless update
     true
@@ -81,7 +83,7 @@ class FeedManager
   # @param [Boolean] update
   # @return [Boolean]
   def push_to_list(list, status, update: false)
-    return false if filter_from_list?(status, list) || !add_to_feed(:list, list.id, status, list.account.user&.aggregates_reblogs?)
+    return false if filter_from_list?(status, list) || !add_to_feed(:list, list.id, status, aggregate_reblogs: list.account.user&.aggregates_reblogs?)
 
     trim(:list, list.id)
     PushUpdateWorker.perform_async(list.account_id, status.id, "timeline:list:#{list.id}", { 'update' => update }) if push_update_required?("timeline:list:#{list.id}")
@@ -94,7 +96,7 @@ class FeedManager
   # @param [Boolean] update
   # @return [Boolean]
   def unpush_from_list(list, status, update: false)
-    return false unless remove_from_feed(:list, list.id, status, list.account.user&.aggregates_reblogs?)
+    return false unless remove_from_feed(:list, list.id, status, aggregate_reblogs: list.account.user&.aggregates_reblogs?)
 
     redis.publish("timeline:list:#{list.id}", Oj.dump(event: :delete, payload: status.id.to_s)) unless update
     true
@@ -120,7 +122,7 @@ class FeedManager
     statuses.each do |status|
       next if filter_from_home?(status, into_account.id, crutches)
 
-      add_to_feed(:home, into_account.id, status, aggregate)
+      add_to_feed(:home, into_account.id, status, aggregate_reblogs: aggregate)
     end
 
     trim(:home, into_account.id)
@@ -146,7 +148,7 @@ class FeedManager
     statuses.each do |status|
       next if filter_from_home?(status, list.account_id, crutches) || filter_from_list?(status, list)
 
-      add_to_feed(:list, list.id, status, aggregate)
+      add_to_feed(:list, list.id, status, aggregate_reblogs: aggregate)
     end
 
     trim(:list, list.id)
@@ -161,7 +163,7 @@ class FeedManager
     timeline_status_ids = redis.zrange(timeline_key, 0, -1)
 
     from_account.statuses.select('id, reblog_of_id').where(id: timeline_status_ids).reorder(nil).find_each do |status|
-      remove_from_feed(:home, into_account.id, status, into_account.user&.aggregates_reblogs?)
+      remove_from_feed(:home, into_account.id, status, aggregate_reblogs: into_account.user&.aggregates_reblogs?)
     end
   end
 
@@ -174,7 +176,7 @@ class FeedManager
     timeline_status_ids = redis.zrange(timeline_key, 0, -1)
 
     from_account.statuses.select('id, reblog_of_id').where(id: timeline_status_ids).reorder(nil).find_each do |status|
-      remove_from_feed(:list, list.id, status, list.account.user&.aggregates_reblogs?)
+      remove_from_feed(:list, list.id, status, aggregate_reblogs: list.account.user&.aggregates_reblogs?)
     end
   end
 
@@ -237,7 +239,7 @@ class FeedManager
     timeline_key = key(:home, account.id)
 
     account.statuses.limit(limit).each do |status|
-      add_to_feed(:home, account.id, status, aggregate)
+      add_to_feed(:home, account.id, status, aggregate_reblogs: aggregate)
     end
 
     account.following.includes(:account_stat).find_each do |target_account|
@@ -257,7 +259,7 @@ class FeedManager
       statuses.each do |status|
         next if filter_from_home?(status, account.id, crutches)
 
-        add_to_feed(:home, account.id, status, aggregate)
+        add_to_feed(:home, account.id, status, aggregate_reblogs: aggregate)
       end
 
       trim(:home, account.id)
@@ -416,6 +418,16 @@ class FeedManager
     false
   end
 
+  # Check if a status should not be added to the home feed when it comes
+  # from a followed hashtag
+  # @param [Status] status
+  # @param [Integer] receiver_id
+  # @param [Hash] crutches
+  # @return [Boolean]
+  def filter_from_tags?(status, receiver_id, crutches)
+    receiver_id != status.account_id && (((crutches[:active_mentions][status.id] || []) + [status.account_id]).any? { |target_account_id| crutches[:blocking][target_account_id] || crutches[:muting][target_account_id] } || crutches[:blocked_by][status.account_id] || crutches[:domain_blocking][status.account.domain])
+  end
+
   # Adds a status to an account's feed, returning true if a status was
   # added, and false if it was not added to the feed. Note that this is
   # an internal helper: callers must call trim or push updates if
@@ -425,7 +437,7 @@ class FeedManager
   # @param [Status] status
   # @param [Boolean] aggregate_reblogs
   # @return [Boolean]
-  def add_to_feed(timeline_type, account_id, status, aggregate_reblogs = true)
+  def add_to_feed(timeline_type, account_id, status, aggregate_reblogs: true)
     timeline_key = key(timeline_type, account_id)
     reblog_key   = key(timeline_type, account_id, 'reblogs')
 
@@ -473,7 +485,7 @@ class FeedManager
   # @param [Status] status
   # @param [Boolean] aggregate_reblogs
   # @return [Boolean]
-  def remove_from_feed(timeline_type, account_id, status, aggregate_reblogs = true)
+  def remove_from_feed(timeline_type, account_id, status, aggregate_reblogs: true)
     timeline_key = key(timeline_type, account_id)
     reblog_key   = key(timeline_type, account_id, 'reblogs')
 
diff --git a/app/models/tag.rb b/app/models/tag.rb
index f078007f2..eebf3b47d 100644
--- a/app/models/tag.rb
+++ b/app/models/tag.rb
@@ -22,13 +22,16 @@ class Tag < ApplicationRecord
   has_and_belongs_to_many :statuses
   has_and_belongs_to_many :accounts
 
+  has_many :passive_relationships, class_name: 'TagFollow', inverse_of: :tag, dependent: :destroy
   has_many :featured_tags, dependent: :destroy, inverse_of: :tag
+  has_many :followers, through: :passive_relationships, source: :account
 
   HASHTAG_SEPARATORS = "_\u00B7\u200c"
   HASHTAG_NAME_RE    = "([[:alnum:]_][[:alnum:]#{HASHTAG_SEPARATORS}]*[[:alpha:]#{HASHTAG_SEPARATORS}][[:alnum:]#{HASHTAG_SEPARATORS}]*[[:alnum:]_])|([[:alnum:]_]*[[:alpha:]][[:alnum:]_]*)"
   HASHTAG_RE         = /(?:^|[^\/\)\w])#(#{HASHTAG_NAME_RE})/i
 
   validates :name, presence: true, format: { with: /\A(#{HASHTAG_NAME_RE})\z/i }
+  validates :display_name, format: { with: /\A(#{HASHTAG_NAME_RE})\z/i }
   validate :validate_name_change, if: -> { !new_record? && name_changed? }
   validate :validate_display_name_change, if: -> { !new_record? && display_name_changed? }
 
@@ -99,7 +102,7 @@ class Tag < ApplicationRecord
       names = Array(name_or_names).map { |str| [normalize(str), str] }.uniq(&:first)
 
       names.map do |(normalized_name, display_name)|
-        tag = matching_name(normalized_name).first || create(name: normalized_name, display_name: display_name)
+        tag = matching_name(normalized_name).first || create(name: normalized_name, display_name: display_name.gsub(/[^[:alnum:]#{HASHTAG_SEPARATORS}]/, ''))
 
         yield tag if block_given?
 
diff --git a/app/models/tag_follow.rb b/app/models/tag_follow.rb
new file mode 100644
index 000000000..abe36cd17
--- /dev/null
+++ b/app/models/tag_follow.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: tag_follows
+#
+#  id         :bigint(8)        not null, primary key
+#  tag_id     :bigint(8)        not null
+#  account_id :bigint(8)        not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class TagFollow < ApplicationRecord
+  include RateLimitable
+  include Paginable
+
+  belongs_to :tag
+  belongs_to :account
+
+  accepts_nested_attributes_for :tag
+
+  rate_limit by: :account, family: :follows
+end
diff --git a/app/presenters/tag_relationships_presenter.rb b/app/presenters/tag_relationships_presenter.rb
new file mode 100644
index 000000000..c3bdbaf07
--- /dev/null
+++ b/app/presenters/tag_relationships_presenter.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class TagRelationshipsPresenter
+  attr_reader :following_map
+
+  def initialize(tags, current_account_id = nil, **options)
+    @following_map = begin
+      if current_account_id.nil?
+        {}
+      else
+        TagFollow.select(:tag_id).where(tag_id: tags.map(&:id), account_id: current_account_id).each_with_object({}) { |f, h| h[f.tag_id] = true }.merge(options[:following_map] || {})
+      end
+    end
+  end
+end
diff --git a/app/serializers/rest/tag_serializer.rb b/app/serializers/rest/tag_serializer.rb
index 52bfaa4ce..7801e77d1 100644
--- a/app/serializers/rest/tag_serializer.rb
+++ b/app/serializers/rest/tag_serializer.rb
@@ -5,6 +5,8 @@ class REST::TagSerializer < ActiveModel::Serializer
 
   attributes :name, :url, :history
 
+  attribute :following, if: :current_user?
+
   def url
     tag_url(object)
   end
@@ -12,4 +14,16 @@ class REST::TagSerializer < ActiveModel::Serializer
   def name
     object.display_name
   end
+
+  def following
+    if instance_options && instance_options[:relationships]
+      instance_options[:relationships].following_map[object.id] || false
+    else
+      TagFollow.where(tag_id: object.id, account_id: current_user.account_id).exists?
+    end
+  end
+
+  def current_user?
+    !current_user.nil?
+  end
 end
diff --git a/app/services/fan_out_on_write_service.rb b/app/services/fan_out_on_write_service.rb
index de5c5ebe4..ce20a146e 100644
--- a/app/services/fan_out_on_write_service.rb
+++ b/app/services/fan_out_on_write_service.rb
@@ -16,6 +16,7 @@ class FanOutOnWriteService < BaseService
     check_race_condition!
 
     fan_out_to_local_recipients!
+    fan_out_to_public_recipients! if broadcastable?
     fan_out_to_public_streams! if broadcastable?
   end
 
@@ -50,6 +51,10 @@ class FanOutOnWriteService < BaseService
     end
   end
 
+  def fan_out_to_public_recipients!
+    deliver_to_hashtag_followers!
+  end
+
   def fan_out_to_public_streams!
     broadcast_to_hashtag_streams!
     broadcast_to_public_streams!
@@ -83,6 +88,14 @@ class FanOutOnWriteService < BaseService
     end
   end
 
+  def deliver_to_hashtag_followers!
+    TagFollow.where(tag_id: @status.tags.map(&:id)).select(:id, :account_id).reorder(nil).find_in_batches do |follows|
+      FeedInsertWorker.push_bulk(follows) do |follow|
+        [@status.id, follow.account_id, 'tags', { 'update' => update? }]
+      end
+    end
+  end
+
   def deliver_to_lists!
     @account.lists_for_local_distribution.select(:id).reorder(nil).find_in_batches do |lists|
       FeedInsertWorker.push_bulk(lists) do |list|
@@ -100,7 +113,7 @@ class FanOutOnWriteService < BaseService
   end
 
   def broadcast_to_hashtag_streams!
-    @status.tags.pluck(:name).each do |hashtag|
+    @status.tags.map(&:name).each do |hashtag|
       redis.publish("timeline:hashtag:#{hashtag.mb_chars.downcase}", anonymous_payload)
       redis.publish("timeline:hashtag:#{hashtag.mb_chars.downcase}:local", anonymous_payload) if @status.local?
     end
diff --git a/app/workers/feed_insert_worker.rb b/app/workers/feed_insert_worker.rb
index 40bc9cb6e..758cebd4b 100644
--- a/app/workers/feed_insert_worker.rb
+++ b/app/workers/feed_insert_worker.rb
@@ -9,7 +9,7 @@ class FeedInsertWorker
     @options   = options.symbolize_keys
 
     case @type
-    when :home
+    when :home, :tags
       @follower = Account.find(id)
     when :list
       @list     = List.find(id)
@@ -36,6 +36,8 @@ class FeedInsertWorker
     case @type
     when :home
       FeedManager.instance.filter?(:home, @status, @follower)
+    when :tags
+      FeedManager.instance.filter?(:tags, @status, @follower)
     when :list
       FeedManager.instance.filter?(:list, @status, @list)
     end
@@ -49,7 +51,7 @@ class FeedInsertWorker
 
   def perform_push
     case @type
-    when :home
+    when :home, :tags
       FeedManager.instance.push_to_home(@follower, @status, update: update?)
     when :list
       FeedManager.instance.push_to_list(@list, @status, update: update?)
@@ -58,7 +60,7 @@ class FeedInsertWorker
 
   def perform_unpush
     case @type
-    when :home
+    when :home, :tags
       FeedManager.instance.unpush_from_home(@follower, @status, update: true)
     when :list
       FeedManager.instance.unpush_from_list(@list, @status, update: true)
diff --git a/config/routes.rb b/config/routes.rb
index 177c1cff4..7a902b1f0 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -530,6 +530,15 @@ Rails.application.routes.draw do
         resource :note, only: :create, controller: 'accounts/notes'
       end
 
+      resources :tags, only: [:show], constraints: { id: /#{Tag::HASHTAG_NAME_RE}/ } do
+        member do
+          post :follow
+          post :unfollow
+        end
+      end
+
+      resources :followed_tags, only: [:index]
+
       resources :lists, only: [:index, :create, :show, :update, :destroy] do
         resource :accounts, only: [:show, :create, :destroy], controller: 'lists/accounts'
       end
diff --git a/db/migrate/20220714171049_create_tag_follows.rb b/db/migrate/20220714171049_create_tag_follows.rb
new file mode 100644
index 000000000..a393e90f5
--- /dev/null
+++ b/db/migrate/20220714171049_create_tag_follows.rb
@@ -0,0 +1,12 @@
+class CreateTagFollows < ActiveRecord::Migration[6.1]
+  def change
+    create_table :tag_follows do |t|
+      t.belongs_to :tag, null: false, foreign_key: { on_delete: :cascade }
+      t.belongs_to :account, null: false, foreign_key: { on_delete: :cascade }, index: false
+
+      t.timestamps
+    end
+
+    add_index :tag_follows, [:account_id, :tag_id], unique: true
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 9b465b674..2263dc7d7 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2022_07_10_102457) do
+ActiveRecord::Schema.define(version: 2022_07_14_171049) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -928,6 +928,15 @@ ActiveRecord::Schema.define(version: 2022_07_10_102457) do
     t.datetime "updated_at", null: false
   end
 
+  create_table "tag_follows", force: :cascade do |t|
+    t.bigint "tag_id", null: false
+    t.bigint "account_id", null: false
+    t.datetime "created_at", precision: 6, null: false
+    t.datetime "updated_at", precision: 6, null: false
+    t.index ["account_id", "tag_id"], name: "index_tag_follows_on_account_id_and_tag_id", unique: true
+    t.index ["tag_id"], name: "index_tag_follows_on_tag_id"
+  end
+
   create_table "tags", force: :cascade do |t|
     t.string "name", default: "", null: false
     t.datetime "created_at", null: false
@@ -1167,6 +1176,8 @@ ActiveRecord::Schema.define(version: 2022_07_10_102457) do
   add_foreign_key "statuses", "statuses", column: "reblog_of_id", on_delete: :cascade
   add_foreign_key "statuses_tags", "statuses", on_delete: :cascade
   add_foreign_key "statuses_tags", "tags", name: "fk_3081861e21", on_delete: :cascade
+  add_foreign_key "tag_follows", "accounts", on_delete: :cascade
+  add_foreign_key "tag_follows", "tags", on_delete: :cascade
   add_foreign_key "tombstones", "accounts", on_delete: :cascade
   add_foreign_key "user_invite_requests", "users", on_delete: :cascade
   add_foreign_key "users", "accounts", name: "fk_50500f500d", on_delete: :cascade
diff --git a/spec/controllers/api/v1/followed_tags_controller_spec.rb b/spec/controllers/api/v1/followed_tags_controller_spec.rb
new file mode 100644
index 000000000..2191350ef
--- /dev/null
+++ b/spec/controllers/api/v1/followed_tags_controller_spec.rb
@@ -0,0 +1,23 @@
+require 'rails_helper'
+
+RSpec.describe Api::V1::FollowedTagsController, type: :controller do
+  render_views
+
+  let(:user)   { Fabricate(:user) }
+  let(:scopes) { 'read:follows' }
+  let(:token)  { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
+
+  before { allow(controller).to receive(:doorkeeper_token) { token } }
+
+  describe 'GET #index' do
+    let!(:tag_follows) { Fabricate.times(5, :tag_follow, account: user.account) }
+
+    before do
+      get :index, params: { limit: 1 }
+    end
+
+    it 'returns http success' do
+      expect(response).to have_http_status(:success)
+    end
+  end
+end
diff --git a/spec/controllers/api/v1/tags_controller_spec.rb b/spec/controllers/api/v1/tags_controller_spec.rb
new file mode 100644
index 000000000..ac42660df
--- /dev/null
+++ b/spec/controllers/api/v1/tags_controller_spec.rb
@@ -0,0 +1,82 @@
+require 'rails_helper'
+
+RSpec.describe Api::V1::TagsController, type: :controller do
+  render_views
+
+  let(:user)   { Fabricate(:user) }
+  let(:scopes) { 'write:follows' }
+  let(:token)  { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
+
+  before { allow(controller).to receive(:doorkeeper_token) { token } }
+
+  describe 'GET #show' do
+    before do
+      get :show, params: { id: name }
+    end
+
+    context 'with existing tag' do
+      let!(:tag) { Fabricate(:tag) }
+      let(:name) { tag.name }
+
+      it 'returns http success' do
+        expect(response).to have_http_status(:success)
+      end
+    end
+
+    context 'with non-existing tag' do
+      let(:name) { 'hoge' }
+
+      it 'returns http success' do
+        expect(response).to have_http_status(:success)
+      end
+    end
+  end
+
+  describe 'POST #follow' do
+    before do
+      post :follow, params: { id: name }
+    end
+
+    context 'with existing tag' do
+      let!(:tag) { Fabricate(:tag) }
+      let(:name) { tag.name }
+
+      it 'returns http success' do
+        expect(response).to have_http_status(:success)
+      end
+
+      it 'creates follow' do
+        expect(TagFollow.where(tag: tag, account: user.account).exists?).to be true
+      end
+    end
+
+    context 'with non-existing tag' do
+      let(:name) { 'hoge' }
+
+      it 'returns http success' do
+        expect(response).to have_http_status(:success)
+      end
+
+      it 'creates follow' do
+        expect(TagFollow.where(tag: Tag.find_by!(name: name), account: user.account).exists?).to be true
+      end
+    end
+  end
+
+  describe 'POST #unfollow' do
+    let!(:tag) { Fabricate(:tag, name: 'foo') }
+    let!(:tag_follow) { Fabricate(:tag_follow, account: user.account, tag: tag) }
+
+    before do
+      post :unfollow, params: { id: tag.name }
+    end
+
+    it 'returns http success' do
+      expect(response).to have_http_status(:success)
+    end
+
+    it 'removes the follow' do
+      expect(TagFollow.where(tag: tag, account: user.account).exists?).to be false
+    end
+  end
+end
diff --git a/spec/fabricators/tag_follow_fabricator.rb b/spec/fabricators/tag_follow_fabricator.rb
new file mode 100644
index 000000000..a2cccb07a
--- /dev/null
+++ b/spec/fabricators/tag_follow_fabricator.rb
@@ -0,0 +1,4 @@
+Fabricator(:tag_follow) do
+  tag
+  account
+end
diff --git a/spec/models/tag_follow_spec.rb b/spec/models/tag_follow_spec.rb
new file mode 100644
index 000000000..50c04d2e4
--- /dev/null
+++ b/spec/models/tag_follow_spec.rb
@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe TagFollow, type: :model do
+end