From 9bf522f2278f6e6d65d4a25803d1e4b0f15395d4 Mon Sep 17 00:00:00 2001
From: David Taylor <david@taylorhq.com>
Date: Fri, 5 Oct 2018 10:23:52 +0100
Subject: [PATCH] FEATURE: Mixed case tagging (#6454)

- By default, behaviour is not changed: tags are made lowercase upon creation and edit.

- If force_lowercase_tags is disabled, then mixed case tags are allowed.

- Tags must remain case-insensitively unique. This is enforced by ActiveRecord and Postgres.

- A migration is added to provide a `UNIQUE` index on `lower(name)`. Migration includes a safety to correct any current tags that do not meet the criteria.

- A `where_name` scope is added to `models/tag.rb`, to allow easy case-insensitive lookups. This is used instead of `Tag.where(name: "blah")`.

- URLs remain lowercase. Mixed case URLs are functional, but have the lowercase equivalent as the canonical.
---
 .../discourse/lib/render-tag.js.es6            |  5 +++--
 .../discourse/routes/tags-show.js.es6          | 13 ++++++++-----
 .../components/mini-tag-chooser.js.es6         |  1 -
 .../select-kit/components/tag-drop.js.es6      |  2 +-
 .../javascripts/select-kit/mixins/tags.js.es6  |  8 ++++++--
 app/controllers/tags_controller.rb             | 10 ++++------
 app/models/tag.rb                              | 11 ++++++++++-
 app/models/tag_user.rb                         |  2 +-
 app/services/search_indexer.rb                 |  2 +-
 config/locales/server.en.yml                   |  1 +
 config/site_settings.yml                       |  2 ++
 db/migrate/20180928105835_add_index_to_tags.rb | 17 +++++++++++++++++
 lib/discourse_tagging.rb                       | 18 +++++++++---------
 lib/search.rb                                  |  8 ++++----
 lib/topic_query.rb                             |  5 +++--
 script/bulk_import/discourse_merger.rb         |  2 +-
 spec/components/discourse_tagging_spec.rb      | 13 +++++++++++--
 spec/components/search_spec.rb                 |  9 +++++++--
 spec/components/topic_query_spec.rb            |  5 +++++
 spec/models/tag_spec.rb                        |  6 ++++++
 test/javascripts/acceptance/tags-test.js.es6   | 13 +++++++++++--
 .../components/mini-tag-chooser-test.js.es6    | 12 +++++++++++-
 .../components/tag-drop-test.js.es6            | 15 +++++++++++++++
 23 files changed, 137 insertions(+), 43 deletions(-)
 create mode 100644 db/migrate/20180928105835_add_index_to_tags.rb

diff --git a/app/assets/javascripts/discourse/lib/render-tag.js.es6 b/app/assets/javascripts/discourse/lib/render-tag.js.es6
index 533058adf08..8420638b3b5 100644
--- a/app/assets/javascripts/discourse/lib/render-tag.js.es6
+++ b/app/assets/javascripts/discourse/lib/render-tag.js.es6
@@ -1,6 +1,7 @@
 export default function renderTag(tag, params) {
   params = params || {};
-  tag = Handlebars.Utils.escapeExpression(tag);
+  const visibleName = Handlebars.Utils.escapeExpression(tag);
+  tag = visibleName.toLowerCase();
   const classes = ["discourse-tag"];
   const tagName = params.tagName || "a";
   let path;
@@ -29,7 +30,7 @@ export default function renderTag(tag, params) {
     " class='" +
     classes.join(" ") +
     "'>" +
-    tag +
+    visibleName +
     "</" +
     tagName +
     ">";
diff --git a/app/assets/javascripts/discourse/routes/tags-show.js.es6 b/app/assets/javascripts/discourse/routes/tags-show.js.es6
index 437f3e4e821..2bd25671571 100644
--- a/app/assets/javascripts/discourse/routes/tags-show.js.es6
+++ b/app/assets/javascripts/discourse/routes/tags-show.js.es6
@@ -49,10 +49,12 @@ export default Discourse.Route.extend({
 
     if (tag && tag.get("id") !== "none" && this.get("currentUser")) {
       // If logged in, we should get the tag's user settings
-      return this.store.find("tagNotification", tag.get("id")).then(tn => {
-        this.set("tagNotification", tn);
-        return tag;
-      });
+      return this.store
+        .find("tagNotification", tag.get("id").toLowerCase())
+        .then(tn => {
+          this.set("tagNotification", tn);
+          return tag;
+        });
     }
 
     return tag;
@@ -67,7 +69,7 @@ export default Discourse.Route.extend({
     const categorySlug = this.get("categorySlug");
     const parentCategorySlug = this.get("parentCategorySlug");
     const filter = this.get("navMode");
-    const tag_id = tag ? tag.id : "none";
+    const tag_id = tag ? tag.id.toLowerCase() : "none";
 
     if (categorySlug) {
       var category = Discourse.Category.findBySlug(
@@ -100,6 +102,7 @@ export default Discourse.Route.extend({
       params,
       {}
     ).then(list => {
+      tag.set("id", list.topic_list.tags[0].name); // Update name of tag (case might be different)
       controller.setProperties({
         list: list,
         canCreateTopic: list.get("can_create_topic"),
diff --git a/app/assets/javascripts/select-kit/components/mini-tag-chooser.js.es6 b/app/assets/javascripts/select-kit/components/mini-tag-chooser.js.es6
index f44fdb33e22..765aae62dde 100644
--- a/app/assets/javascripts/select-kit/components/mini-tag-chooser.js.es6
+++ b/app/assets/javascripts/select-kit/components/mini-tag-chooser.js.es6
@@ -225,7 +225,6 @@ export default ComboBox.extend(TagsMixin, {
       case "string":
         // See lib/discourse_tagging#clean_tag.
         return content
-          .toLowerCase()
           .trim()
           .replace(/\s+/, "-")
           .replace(/[\/\?#\[\]@!\$&'\(\)\*\+,;=\.%\\`^\s|\{\}"<>]+/, "")
diff --git a/app/assets/javascripts/select-kit/components/tag-drop.js.es6 b/app/assets/javascripts/select-kit/components/tag-drop.js.es6
index b1bb9799602..c99806b8481 100644
--- a/app/assets/javascripts/select-kit/components/tag-drop.js.es6
+++ b/app/assets/javascripts/select-kit/components/tag-drop.js.es6
@@ -145,7 +145,7 @@ export default ComboBoxComponent.extend(TagsMixin, {
       if (this.get("currentCategory")) {
         url += this.get("currentCategory.url");
       }
-      url = `${url}/${tagId}`;
+      url = `${url}/${tagId.toLowerCase()}`;
       DiscourseURL.routeTo(url);
     },
 
diff --git a/app/assets/javascripts/select-kit/mixins/tags.js.es6 b/app/assets/javascripts/select-kit/mixins/tags.js.es6
index 70da1641811..0f0bef09ad4 100644
--- a/app/assets/javascripts/select-kit/mixins/tags.js.es6
+++ b/app/assets/javascripts/select-kit/mixins/tags.js.es6
@@ -52,12 +52,16 @@ export default Ember.Mixin.create({
       return false;
     }
 
+    const toLowerCaseOrUndefined = string => {
+      return string === undefined ? undefined : string.toLowerCase();
+    };
+
     const inCollection = this.get("collectionComputedContent")
-      .map(c => get(c, "id"))
+      .map(c => toLowerCaseOrUndefined(get(c, "id")))
       .includes(term);
 
     const inSelection = this.get("selection")
-      .map(s => get(s, "value").toLowerCase())
+      .map(s => toLowerCaseOrUndefined(get(s, "value")))
       .includes(term);
     if (inCollection || inSelection) {
       return false;
diff --git a/app/controllers/tags_controller.rb b/app/controllers/tags_controller.rb
index fccddd0e4ab..8c7c854e7c6 100644
--- a/app/controllers/tags_controller.rb
+++ b/app/controllers/tags_controller.rb
@@ -89,7 +89,7 @@ class TagsController < ::ApplicationController
       path_name = url_method(params.slice(:category, :parent_category))
       canonical_url "#{Discourse.base_url_no_prefix}#{public_send(path_name, *(params.slice(:parent_category, :category, :tag_id).values.map { |t| t.force_encoding("UTF-8") }))}"
 
-      if @list.topics.size == 0 && params[:tag_id] != 'none' && !Tag.where(name: @tag_id).exists?
+      if @list.topics.size == 0 && params[:tag_id] != 'none' && !Tag.where_name(@tag_id).exists?
         raise Discourse::NotFound.new("tag not found", check_permalinks: true)
       else
         respond_with_list(@list)
@@ -162,7 +162,7 @@ class TagsController < ::ApplicationController
 
     json_response = { results: tags }
 
-    if Tag.where(name: params[:q]).exists? && !tags.find { |h| h[:id] == params[:q] }
+    if Tag.where_name(params[:q]).exists? && !tags.find { |h| h[:id] == params[:q] }
       # filter_allowed_tags determined that the tag entered is not allowed
       json_response[:forbidden] = params[:q]
     end
@@ -171,7 +171,7 @@ class TagsController < ::ApplicationController
   end
 
   def notifications
-    tag = Tag.find_by_name(params[:tag_id])
+    tag = Tag.where_name(params[:tag_id]).first
     raise Discourse::NotFound unless tag
     level = tag.tag_users.where(user: current_user).first.try(:notification_level) || TagUser.notification_levels[:regular]
     render json: { tag_notification: { id: tag.name, notification_level: level.to_i } }
@@ -186,9 +186,7 @@ class TagsController < ::ApplicationController
   end
 
   def check_hashtag
-    tag_values = params[:tag_values].each(&:downcase!)
-
-    valid_tags = Tag.where(name: tag_values).map do |tag|
+    valid_tags = Tag.where_name(params[:tag_values]).map do |tag|
       { value: tag.name, url: tag.full_url }
     end.compact
 
diff --git a/app/models/tag.rb b/app/models/tag.rb
index 11002e25417..f2ca7f067e3 100644
--- a/app/models/tag.rb
+++ b/app/models/tag.rb
@@ -2,7 +2,12 @@ class Tag < ActiveRecord::Base
   include Searchable
   include HasDestroyedWebHook
 
-  validates :name, presence: true, uniqueness: true
+  validates :name, presence: true, uniqueness: { case_sensitive: false }
+
+  scope :where_name, ->(name) do
+    name = Array(name).map(&:downcase)
+    where("lower(name) IN (?)", name)
+  end
 
   has_many :tag_users # notification settings
 
@@ -59,6 +64,10 @@ class Tag < ActiveRecord::Base
     SQL
   end
 
+  def self.find_by_name(name)
+    self.find_by('lower(name) = ?', name.downcase)
+  end
+
   def self.top_tags(limit_arg: nil, category: nil, guardian: nil)
     limit = limit_arg || SiteSetting.max_tags_in_filter_list
     scope_category_ids = (guardian || Guardian.new).allowed_category_ids
diff --git a/app/models/tag_user.rb b/app/models/tag_user.rb
index 87eafe5404d..bee895bfe0e 100644
--- a/app/models/tag_user.rb
+++ b/app/models/tag_user.rb
@@ -19,7 +19,7 @@ class TagUser < ActiveRecord::Base
     records = TagUser.where(user: user, notification_level: notification_levels[level])
     old_ids = records.pluck(:tag_id)
 
-    tag_ids = tags.empty? ? [] : Tag.where('name in (?)', tags).pluck(:id)
+    tag_ids = tags.empty? ? [] : Tag.where_name(tags).pluck(:id)
 
     remove = (old_ids - tag_ids)
     if remove.present?
diff --git a/app/services/search_indexer.rb b/app/services/search_indexer.rb
index 72240873685..e31bf8e4914 100644
--- a/app/services/search_indexer.rb
+++ b/app/services/search_indexer.rb
@@ -105,7 +105,7 @@ class SearchIndexer
   end
 
   def self.update_tags_index(tag_id, name)
-    update_index(table: 'tag', id: tag_id, raw_data: [name])
+    update_index(table: 'tag', id: tag_id, raw_data: [name.downcase])
   end
 
   def self.queue_post_reindex(topic_id)
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 1ea6b56ab1b..e5236174f00 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -1798,6 +1798,7 @@ en:
     min_trust_level_to_tag_topics: "Minimum trust level required to tag topics"
     suppress_overlapping_tags_in_list: "If tags match exact words in topic titles, don't show the tag"
     remove_muted_tags_from_latest: "Don't show topics tagged with muted tags in the latest topic list."
+    force_lowercase_tags: "Force all new tags to be entirely lowercase."
 
     company_short_name: "Company Name (short)"
     company_full_name: "Company Name (full)"
diff --git a/config/site_settings.yml b/config/site_settings.yml
index f82095adb28..6b932a524c6 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -1788,3 +1788,5 @@ tags:
     client: true
   remove_muted_tags_from_latest:
     default: false
+  force_lowercase_tags:
+    default: true
\ No newline at end of file
diff --git a/db/migrate/20180928105835_add_index_to_tags.rb b/db/migrate/20180928105835_add_index_to_tags.rb
new file mode 100644
index 00000000000..50a45870668
--- /dev/null
+++ b/db/migrate/20180928105835_add_index_to_tags.rb
@@ -0,0 +1,17 @@
+class AddIndexToTags < ActiveRecord::Migration[5.2]
+  def up
+    # Append ID to any tags that already have duplicate names
+    # Super rare case, as this is not possible to do via the UI
+    # Might affect some imports
+    execute <<~SQL
+      UPDATE tags
+      SET name = name || id
+      WHERE EXISTS(SELECT * FROM tags t WHERE lower(t.name) = lower(tags.name) AND t.id < tags.id)
+    SQL
+
+    add_index :tags, 'lower(name)', unique: true
+  end
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end
diff --git a/lib/discourse_tagging.rb b/lib/discourse_tagging.rb
index 030927df29e..2c0519cd199 100644
--- a/lib/discourse_tagging.rb
+++ b/lib/discourse_tagging.rb
@@ -39,7 +39,7 @@ module DiscourseTagging
         # guardian is explicitly nil cause we don't want to strip all
         # staff tags that already passed validation
         tags = filter_allowed_tags(
-          Tag.where(name: tag_names),
+          Tag.where_name(tag_names),
           nil, # guardian
           for_topic: true,
           category: category,
@@ -48,7 +48,7 @@ module DiscourseTagging
 
         if tags.size < tag_names.size && (category.nil? || (category.tags.count == 0 && category.tag_groups.count == 0))
           tag_names.each do |name|
-            unless Tag.where(name: name).exists?
+            unless Tag.where_name(name).exists?
               tags << Tag.create(name: name)
             end
           end
@@ -82,8 +82,7 @@ module DiscourseTagging
   #   for_topic: results are for tagging a topic
   #   selected_tags: an array of tag names that are in the current selection
   def self.filter_allowed_tags(query, guardian, opts = {})
-
-    selected_tag_ids = opts[:selected_tags] ? Tag.where(name: opts[:selected_tags]).pluck(:id) : []
+    selected_tag_ids = opts[:selected_tags] ? Tag.where_name(opts[:selected_tags]).pluck(:id) : []
 
     if !opts[:for_topic] && !selected_tag_ids.empty?
       query = query.where('tags.id NOT IN (?)', selected_tag_ids)
@@ -92,8 +91,8 @@ module DiscourseTagging
     term = opts[:term]
     if term.present?
       term.gsub!("_", "\\_")
-      term = clean_tag(term)
-      query = query.where('tags.name like ?', "%#{term}%")
+      term = clean_tag(term).downcase
+      query = query.where('lower(tags.name) like ?', "%#{term}%")
     end
 
     # Filters for category-specific tags:
@@ -203,7 +202,8 @@ module DiscourseTagging
   end
 
   def self.clean_tag(tag)
-    tag.downcase.strip
+    tag.downcase! if SiteSetting.force_lowercase_tags
+    tag.strip
       .gsub(/\s+/, '-').squeeze('-')
       .gsub(TAGS_FILTER_REGEXP, '')[0...SiteSetting.max_tag_length]
   end
@@ -212,7 +212,7 @@ module DiscourseTagging
 
     return [] unless guardian.can_tag_topics? && tags_arg.present?
 
-    tag_names = Tag.where(name: tags_arg).pluck(:name)
+    tag_names = Tag.where_name(tags_arg).pluck(:name)
 
     if guardian.can_create_tag?
       tag_names += (tags_arg - tag_names).map { |t| clean_tag(t) }
@@ -226,7 +226,7 @@ module DiscourseTagging
   def self.add_or_create_tags_by_name(taggable, tag_names_arg, opts = {})
     tag_names = DiscourseTagging.tags_for_saving(tag_names_arg, Guardian.new(Discourse.system_user), opts) || []
     if taggable.tags.pluck(:name).sort != tag_names.sort
-      taggable.tags = Tag.where(name: tag_names).all
+      taggable.tags = Tag.where_name(tag_names).all
       if taggable.tags.size < tag_names.size
         new_tag_names = tag_names - taggable.tags.map(&:name)
         new_tag_names.each do |name|
diff --git a/lib/search.rb b/lib/search.rb
index 80c6df50532..d61833e9996 100644
--- a/lib/search.rb
+++ b/lib/search.rb
@@ -407,7 +407,7 @@ class Search
       posts.where("topics.category_id IN (?)", category_ids)
     else
       # try a possible tag match
-      tag_id = Tag.where(name: slug[0]).pluck(:id).first
+      tag_id = Tag.where_name(slug[0]).pluck(:id).first
       if (tag_id)
         posts.where("topics.id IN (
           SELECT DISTINCT(tt.topic_id)
@@ -496,7 +496,7 @@ class Search
 
   def search_tags(posts, match, positive:)
     return if match.nil?
-
+    match.downcase!
     modifier = positive ? "" : "NOT"
 
     if match.include?('+')
@@ -507,7 +507,7 @@ class Search
         FROM topic_tags tt, tags
         WHERE tt.tag_id = tags.id
         GROUP BY tt.topic_id
-        HAVING to_tsvector(#{default_ts_config}, array_to_string(array_agg(tags.name), ' ')) @@ to_tsquery(#{default_ts_config}, ?)
+        HAVING to_tsvector(#{default_ts_config}, array_to_string(array_agg(lower(tags.name)), ' ')) @@ to_tsquery(#{default_ts_config}, ?)
       )", tags.join('&'))
     else
       tags = match.split(",")
@@ -515,7 +515,7 @@ class Search
       posts.where("topics.id #{modifier} IN (
         SELECT DISTINCT(tt.topic_id)
         FROM topic_tags tt, tags
-        WHERE tt.tag_id = tags.id AND tags.name IN (?)
+        WHERE tt.tag_id = tags.id AND lower(tags.name) IN (?)
       )", tags)
     end
   end
diff --git a/lib/topic_query.rb b/lib/topic_query.rb
index 202d3444704..6e299e73ba6 100644
--- a/lib/topic_query.rb
+++ b/lib/topic_query.rb
@@ -634,11 +634,12 @@ class TopicQuery
       result = result.preload(:tags)
 
       if @options[:tags] && @options[:tags].size > 0
+        @options[:tags].each { |t| t.downcase! if t.is_a? String }
 
         if @options[:match_all_tags]
           # ALL of the given tags:
           tags_count = @options[:tags].length
-          @options[:tags] = Tag.where(name: @options[:tags]).pluck(:id) unless @options[:tags][0].is_a?(Integer)
+          @options[:tags] = Tag.where_name(@options[:tags]).pluck(:id) unless @options[:tags][0].is_a?(Integer)
 
           if tags_count == @options[:tags].length
             @options[:tags].each_with_index do |tag, index|
@@ -654,7 +655,7 @@ class TopicQuery
           if @options[:tags][0].is_a?(Integer)
             result = result.where("tags.id in (?)", @options[:tags])
           else
-            result = result.where("tags.name in (?)", @options[:tags])
+            result = result.where("lower(tags.name) in (?)", @options[:tags])
           end
         end
       elsif @options[:no_tags]
diff --git a/script/bulk_import/discourse_merger.rb b/script/bulk_import/discourse_merger.rb
index 3d0f4131c7e..2a44a343fa5 100644
--- a/script/bulk_import/discourse_merger.rb
+++ b/script/bulk_import/discourse_merger.rb
@@ -271,7 +271,7 @@ class BulkImport::DiscourseMerger < BulkImport::Base
     @raw_connection.copy_data(sql, @encoder) do
       source_raw_connection.exec("SELECT #{columns.map { |c| "\"#{c}\"" }.join(', ')} FROM tags").each do |row|
 
-        if existing = Tag.where(name: row['name']).first
+        if existing = Tag.where_name(row['name']).first
           @tags[row['id']] = existing.id
           next
         end
diff --git a/spec/components/discourse_tagging_spec.rb b/spec/components/discourse_tagging_spec.rb
index c378699e893..5d52375406b 100644
--- a/spec/components/discourse_tagging_spec.rb
+++ b/spec/components/discourse_tagging_spec.rb
@@ -13,7 +13,7 @@ describe DiscourseTagging do
 
   let!(:tag1) { Fabricate(:tag, name: "fun") }
   let!(:tag2) { Fabricate(:tag, name: "fun2") }
-  let!(:tag3) { Fabricate(:tag, name: "fun3") }
+  let!(:tag3) { Fabricate(:tag, name: "Fun3") }
 
   before do
     SiteSetting.tagging_enabled = true
@@ -186,7 +186,8 @@ describe DiscourseTagging do
 
       it "returns only existing tag names" do
         Fabricate(:tag, name: 'oldtag')
-        expect(described_class.tags_for_saving(['newtag', 'oldtag'], guardian).try(:sort)).to eq(['oldtag'])
+        Fabricate(:tag, name: 'oldTag2')
+        expect(described_class.tags_for_saving(['newtag', 'oldtag', 'oldtag2'], guardian)).to contain_exactly('oldtag', 'oldTag2')
       end
     end
 
@@ -205,5 +206,13 @@ describe DiscourseTagging do
         expect(described_class.tags_for_saving(['math=fun', 'fun*2@gmail.com'], guardian).try(:sort)).to eq(['math=fun', 'fun2gmailcom'].sort)
       end
     end
+
+    describe "clean_tag" do
+      it "downcases new tags if setting enabled" do
+        expect(DiscourseTagging.clean_tag("HeLlO")).to eq("hello")
+        SiteSetting.force_lowercase_tags = false
+        expect(DiscourseTagging.clean_tag("HeLlO")).to eq("HeLlO")
+      end
+    end
   end
 end
diff --git a/spec/components/search_spec.rb b/spec/components/search_spec.rb
index 912415a4a12..d987571be33 100644
--- a/spec/components/search_spec.rb
+++ b/spec/components/search_spec.rb
@@ -407,6 +407,7 @@ describe Search do
     end
 
     let!(:tag) { Fabricate(:tag) }
+    let!(:uppercase_tag) { Fabricate(:tag, name: "HeLlO") }
     let(:tag_group) { Fabricate(:tag_group) }
     let(:category) { Fabricate(:category) }
 
@@ -415,7 +416,7 @@ describe Search do
         SiteSetting.tagging_enabled = true
 
         post = Fabricate(:post, raw: 'I am special post')
-        DiscourseTagging.tag_topic_by_names(post.topic, Guardian.new(Fabricate.build(:admin)), [tag.name])
+        DiscourseTagging.tag_topic_by_names(post.topic, Guardian.new(Fabricate.build(:admin)), [tag.name, uppercase_tag.name])
         post.topic.save
 
         # we got to make this index (it is deferred)
@@ -424,6 +425,9 @@ describe Search do
         result = Search.execute(tag.name)
         expect(result.posts.length).to eq(1)
 
+        result = Search.execute("hElLo")
+        expect(result.posts.length).to eq(1)
+
         SiteSetting.tagging_enabled = false
 
         result = Search.execute(tag.name)
@@ -822,9 +826,10 @@ describe Search do
       expect(Search.execute("sams post #sub-category").posts.length).to eq(1)
 
       # tags
-      topic.tags = [Fabricate(:tag, name: 'alpha'), Fabricate(:tag, name: 'привет')]
+      topic.tags = [Fabricate(:tag, name: 'alpha'), Fabricate(:tag, name: 'привет'), Fabricate(:tag, name: 'HeLlO')]
       expect(Search.execute('this is a test #alpha').posts.map(&:id)).to eq([post.id])
       expect(Search.execute('this is a test #привет').posts.map(&:id)).to eq([post.id])
+      expect(Search.execute('this is a test #hElLo').posts.map(&:id)).to eq([post.id])
       expect(Search.execute('this is a test #beta').posts.size).to eq(0)
     end
 
diff --git a/spec/components/topic_query_spec.rb b/spec/components/topic_query_spec.rb
index 08515a3e9da..7eac899b2ba 100644
--- a/spec/components/topic_query_spec.rb
+++ b/spec/components/topic_query_spec.rb
@@ -160,6 +160,7 @@ describe TopicQuery do
   context 'tag filter' do
     let(:tag)       { Fabricate(:tag) }
     let(:other_tag) { Fabricate(:tag) }
+    let(:uppercase_tag) { Fabricate(:tag, name: "HeLlO") }
 
     before do
       SiteSetting.tagging_enabled = true
@@ -169,6 +170,7 @@ describe TopicQuery do
       let!(:tagged_topic1) { Fabricate(:topic, tags: [tag]) }
       let!(:tagged_topic2) { Fabricate(:topic, tags: [other_tag]) }
       let!(:tagged_topic3) { Fabricate(:topic, tags: [tag, other_tag]) }
+      let!(:tagged_topic4) { Fabricate(:topic, tags: [uppercase_tag]) }
       let!(:no_tags_topic) { Fabricate(:topic) }
 
       it "returns topics with the tag when filtered to it" do
@@ -186,6 +188,9 @@ describe TopicQuery do
 
         expect(TopicQuery.new(moderator, tags: [tag.id, other_tag.id]).list_latest.topics)
           .to contain_exactly(tagged_topic1, tagged_topic2, tagged_topic3)
+
+        expect(TopicQuery.new(moderator, tags: ["hElLo"]).list_latest.topics)
+          .to contain_exactly(tagged_topic4)
       end
 
       it "can return topics with all specified tags" do
diff --git a/spec/models/tag_spec.rb b/spec/models/tag_spec.rb
index 69dd546e8da..0fcb28e42a9 100644
--- a/spec/models/tag_spec.rb
+++ b/spec/models/tag_spec.rb
@@ -27,6 +27,12 @@ describe Tag do
       expect(event[:event_name]).to eq(:tag_created)
       expect(event[:params].first).to eq(subject)
     end
+
+    it 'prevents case-insensitive duplicates' do
+      Fabricate.build(:tag, name: "hello").save!
+      expect { Fabricate.build(:tag, name: "hElLo").save! }.to raise_error(ActiveRecord::RecordInvalid)
+    end
+
   end
 
   describe 'destroy' do
diff --git a/test/javascripts/acceptance/tags-test.js.es6 b/test/javascripts/acceptance/tags-test.js.es6
index ced627cce31..b32fabca8aa 100644
--- a/test/javascripts/acceptance/tags-test.js.es6
+++ b/test/javascripts/acceptance/tags-test.js.es6
@@ -32,7 +32,7 @@ QUnit.test("list the tags in groups", async assert => {
               id: 2,
               name: "Ford Cars",
               tags: [
-                { id: "escort", text: "escort", count: 1, pm_count: 0 },
+                { id: "Escort", text: "Escort", count: 1, pm_count: 0 },
                 { id: "focus", text: "focus", count: 3, pm_count: 0 }
               ]
             },
@@ -79,7 +79,16 @@ QUnit.test("list the tags in groups", async assert => {
       .map(i => {
         return $(i).text();
       }),
-    ["focus", "escort"],
+    ["focus", "Escort"],
     "shows the tags in default sort (by count)"
   );
+  assert.deepEqual(
+    $(".tag-list:first .discourse-tag")
+      .toArray()
+      .map(i => {
+        return $(i).attr("href");
+      }),
+    ["/tags/focus", "/tags/escort"],
+    "always uses lowercase URLs for mixed case tags"
+  );
 });
diff --git a/test/javascripts/components/mini-tag-chooser-test.js.es6 b/test/javascripts/components/mini-tag-chooser-test.js.es6
index 69eae80f9ef..d08379be6c1 100644
--- a/test/javascripts/components/mini-tag-chooser-test.js.es6
+++ b/test/javascripts/components/mini-tag-chooser-test.js.es6
@@ -28,7 +28,7 @@ componentTest("default", {
         });
       }
 
-      if (params.queryParams.q === "joffrey" || params.queryParams.q === "invalid'tag" || params.queryParams.q === "01234567890123456789012345") {
+      if (params.queryParams.q.toLowerCase() === "joffrey" || params.queryParams.q === "invalid'tag" || params.queryParams.q === "01234567890123456789012345") {
         return response({results: []});
       }
 
@@ -74,6 +74,16 @@ componentTest("default", {
       "it creates the tag"
     );
 
+    await this.get("subject").expand();
+    await this.get("subject").fillInFilter("Joffrey");
+    await this.get("subject").keyboard("enter");
+    await this.get("subject").collapse();
+    assert.deepEqual(
+      this.get("tags"),
+      ["jeff", "neil", "arpit", "régis", "joffrey"],
+      "it does not allow case insensitive duplicate tags"
+    );
+
     await this.get("subject").expand();
     await this.get("subject").fillInFilter("invalid'tag");
     await this.get("subject").keyboard("enter");
diff --git a/test/javascripts/components/tag-drop-test.js.es6 b/test/javascripts/components/tag-drop-test.js.es6
index e3e8926a1f6..5859f845c73 100644
--- a/test/javascripts/components/tag-drop-test.js.es6
+++ b/test/javascripts/components/tag-drop-test.js.es6
@@ -1,4 +1,5 @@
 import componentTest from "helpers/component-test";
+import DiscourseURL from "discourse/lib/url";
 
 moduleForComponent("tag-drop", {
   integration: true,
@@ -26,6 +27,12 @@ componentTest("default", {
             { "id": "régis", "name": "régis", "count": 2, "pm_count": 0 }
           ]
         });
+      }else if (params.queryParams.q === "dav") {
+        return response({
+          "results": [
+            { "id": "David", "name": "David", "count": 2, "pm_count": 0 }
+          ]
+        });
       }
     });
   },
@@ -66,5 +73,13 @@ componentTest("default", {
       "jeff",
       "it returns top tags for an empty search"
     );
+
+    sandbox.stub(DiscourseURL, "routeTo");
+    await this.get("subject").fillInFilter("dav");
+    await this.get("subject").keyboard("enter");
+    assert.ok(
+      DiscourseURL.routeTo.calledWith("/tags/david"),
+      "it uses lowercase URLs for tags"
+    );
   }
 });