First pass

app/assets/javascripts/discourse/routes/app-route-map.js.es6

@@ -132,6 +132,7 @@ export default function() {
       this.route('showCategory' + filter.capitalize(), {path: '/c/:category/:tag_id/l/' + filter});
       this.route('showParentCategory' + filter.capitalize(), {path: '/c/:parent_category/:category/:tag_id/l/' + filter});
     });
+    this.route('show', {path: 'intersection/:tag_id/:secondary_tag_id'});
   });
 
   this.resource('tagGroups', {path: '/tag_groups'}, function() {

app/assets/javascripts/discourse/routes/tags-show.js.es6

@@ -14,6 +14,8 @@ export default Discourse.Route.extend({
     var tag = this.store.createRecord("tag", { id: Handlebars.Utils.escapeExpression(params.tag_id) }),
         f = '';
 
+    this.set("secondaryTagId", params.secondary_tag_id)
+
     if (params.category) {
       f = 'c/';
       if (params.parent_category) { f += params.parent_category + '/'; }
@@ -43,6 +45,7 @@ export default Discourse.Route.extend({
     const params = controller.getProperties('order', 'ascending');
 
     const categorySlug = this.get('categorySlug');
+    const secondaryTagId = this.get('secondaryTagId')
     const parentCategorySlug = this.get('parentCategorySlug');
     const filter = this.get('navMode');
     const tag_id = (tag ? tag.id : 'none');
@@ -56,6 +59,9 @@ export default Discourse.Route.extend({
       }
 
       this.set('category', category);
+    } else if (secondaryTagId) {
+      params.filter = `tags/intersection/${tag_id}/${secondaryTagId}`;
+      this.set('category', null);
     } else {
       params.filter = `tags/${tag_id}/l/${filter}`;
       this.set('category', null);

app/controllers/tags_controller.rb

@@ -44,6 +44,7 @@ class TagsController < ::ApplicationController
   Discourse.filters.each do |filter|
     define_method("show_#{filter}") do
       @tag_id = DiscourseTagging.clean_tag(params[:tag_id])
+      @secondary_tag_id = DiscourseTagging.clean_tag(params[:secondary_tag_id]) if params[:secondary_tag_id]
 
       page = params[:page].to_i
       list_opts = build_topic_list_options
@@ -71,7 +72,7 @@ class TagsController < ::ApplicationController
       @list.more_topics_url = construct_url_with(:next, list_opts)
       @list.prev_topics_url = construct_url_with(:prev, list_opts)
       @rss = "tag"
-      @description_meta = I18n.t("rss_by_tag", tag: @tag_id)
+      @description_meta = I18n.t("rss_by_tag", tag: [@tag_id, @secondary_tag_id].compact.join(' & '))
       @title = @description_meta
 
       canonical_url "#{Discourse.base_url_no_prefix}#{public_send(url_method(params.slice(:category, :parent_category)))}"
@@ -297,7 +298,8 @@ class TagsController < ::ApplicationController
       if params[:tag_id] == 'none'
         options[:no_tags] = true
       else
-        options[:tags] = [params[:tag_id]]
+        options[:tags] = [params[:tag_id], params[:secondary_tag_id]].compact
+        options[:match_all_tags] = true
       end
 
       options

config/routes.rb

@@ -630,6 +630,7 @@ Discourse::Application.routes.draw do
       get '/:tag_id' => 'tags#show', as: 'tag_show'
       get '/c/:category/:tag_id' => 'tags#show', as: 'tag_category_show'
       get '/c/:parent_category/:category/:tag_id' => 'tags#show', as: 'tag_parent_category_category_show'
+      get '/intersection/:tag_id/:secondary_tag_id' => 'tags#show', as: 'tag_intersection'
       get '/:tag_id/notifications' => 'tags#notifications'
       put '/:tag_id/notifications' => 'tags#update_notifications'
       put '/:tag_id' => 'tags#update'

lib/topic_query.rb

@@ -20,6 +20,7 @@ class TopicQuery
                      visible
                      category
                      tags
+                     match_all_tags
                      no_tags
                      order
                      ascending
@@ -460,11 +461,21 @@ class TopicQuery
 
         if @options[:tags] && @options[:tags].size > 0
           result = result.joins(:tags)
-          # ANY of the given tags:
-          if @options[:tags][0].is_a?(Integer)
-            result = result.where("tags.id in (?)", @options[:tags])
+
+          # ALL of the given tags:
+          if @options[:match_all_tags]
+            @options[:tags] = Tag.where(name: @options[:tags]).pluck(:id) unless @options[:tags][0].is_a?(Integer)
+            @options[:tags].each_with_index do |tag, index|
+              sql_alias = ['t', index].join
+              result = result.joins("INNER JOIN topic_tags #{sql_alias} ON #{sql_alias}.topic_id = topics.id AND #{sql_alias}.tag_id = #{tag}")
+            end
           else
-            result = result.where("tags.name in (?)", @options[:tags])
+            # ANY of the given tags:
+            if @options[:tags][0].is_a?(Integer)
+              result = result.where("tags.id in (?)", @options[:tags])
+            else
+              result = result.where("tags.name in (?)", @options[:tags])
+            end
           end
         elsif @options[:no_tags]
           # the following will do: ("topics"."id" NOT IN (SELECT DISTINCT "topic_tags"."topic_id" FROM "topic_tags"))

spec/components/topic_query_spec.rb

@@ -145,6 +145,10 @@ describe TopicQuery do
         # expect(TopicQuery.new(moderator, tags: [tag.id, other_tag.id]).list_latest.topics.map(&:id)).to eq([tagged_topic3.id].sort)
       end
 
+      it "can return topics with all specified tags" do
+        expect(TopicQuery.new(moderator, tags: [tag.name, other_tag.name], match_all_tags: true).list_latest.topics.map(&:id)).to eq([tagged_topic3.id])
+      end
+
       it "can return topics with no tags" do
         expect(TopicQuery.new(moderator, no_tags: true).list_latest.topics.map(&:id)).to eq([no_tags_topic.id])
       end

spec/controllers/tags_controller_spec.rb

@@ -3,9 +3,13 @@ require 'rails_helper'
 describe TagsController do
   describe 'show_latest' do
     let(:tag)         { Fabricate(:tag) }
+    let(:other_tag)   { Fabricate(:tag) }
     let(:category)    { Fabricate(:category) }
     let(:subcategory) { Fabricate(:category, parent_category_id: category.id) }
 
+    let(:single_tag_topic) { Fabricate(:topic, tags: [tag]) }
+    let(:multi_tag_topic)  { Fabricate(:topic, tags: [tag, other_tag]) }
+
     context 'tagging disabled' do
       it "returns 404" do
         xhr :get, :show_latest, tag_id: tag.name
@@ -23,6 +27,14 @@ describe TagsController do
         expect(response).to be_success
       end
 
+      it "can filter by multiple tags" do
+        single_tag_topic; multi_tag_topic
+        xhr :get, :show_latest, tag_id: tag.name, secondary_tag_id: other_tag.name
+        expect(response).to be_success
+        expect(assigns(:list).topics).to include multi_tag_topic
+        expect(assigns(:list).topics).to_not include single_tag_topic
+      end
+
       it "can filter by category and tag" do
         xhr :get, :show_latest, tag_id: tag.name, category: category.slug
         expect(response).to be_success