DEV: Update experimental /filter route with categories support (#20911)

On the `/filter` route, the categories filtering query language is now
supported in the input per the example provided below:

```
category:bug => topics in the bug category AND all subcategories
=category:bug => topics in the bug category excluding subcategories
category:bug,feature => allow for categories either in bug or feature
=category:bug,feature => allow for exact categories match excluding sub cats
categories: => alias for category
```

Currently composing multiple category filters is not supported as we
have yet to determine what behaviour it should result in. For example,
`category:bug category:feature` would now return topics that are in both
the `bug` and `feature` category but it is not possible for a topic to
belong to two categories.

lib/topics_filter.rb

@@ -9,14 +9,18 @@ class TopicsFilter
   def filter_from_query_string(query_string)
     return @scope if query_string.blank?
 
-    query_string.scan(/(?<exclude>-)?(?<key>\w+):(?<value>[^:\s]+)/) do |exclude, key, value|
+    query_string.scan(
+      /(?<key_prefix>[-=])?(?<key>\w+):(?<value>[^:\s]+)/,
+    ) do |key_prefix, key, value|
       case key
       when "status"
         @scope = filter_status(status: value)
       when "tags"
         value.scan(
-          /^(?<tags>([a-zA-Z0-9\-]+)(?<delimiter>[,+])?([a-zA-Z0-9\-]+)?(\k<delimiter>[a-zA-Z0-9\-]+)*)$/,
+          /^(?<tag_names>([a-zA-Z0-9\-]+)(?<delimiter>[,+])?([a-zA-Z0-9\-]+)?(\k<delimiter>[a-zA-Z0-9\-]+)*)$/,
-        ) do |value, delimiter|
+        ) do |tag_names, delimiter|
+          break if key_prefix && key_prefix != "-"
+
           match_all =
             if delimiter == ","
               false
@@ -25,7 +29,23 @@ class TopicsFilter
             end
 
           @scope =
-            filter_tags(tag_names: value.split(delimiter), exclude: exclude, match_all: match_all)
+            filter_tags(
+              tag_names: tag_names.split(delimiter),
+              exclude: key_prefix.presence,
+              match_all: match_all,
+            )
+        end
+      when "category", "categories"
+        value.scan(
+          /^(?<category_slugs>([a-zA-Z0-9\-]+)(?<delimiter>[,])?([a-zA-Z0-9\-]+)?(\k<delimiter>[a-zA-Z0-9\-]+)*)$/,
+        ) do |category_slugs, delimiter|
+          break if key_prefix && key_prefix != "="
+
+          @scope =
+            filter_categories(
+              category_slugs: category_slugs.split(delimiter),
+              exclude_subcategories: key_prefix.presence,
+            )
         end
       end
     end
@@ -60,6 +80,22 @@ class TopicsFilter
 
   private
 
+  def filter_categories(category_slugs:, exclude_subcategories: false)
+    category_ids =
+      Category
+        .where(slug: category_slugs)
+        .filter { |category| @guardian.can_see_category?(category) }
+        .map(&:id)
+
+    return @scope.none if category_ids.length != category_slugs.length
+
+    if !exclude_subcategories
+      category_ids = category_ids.flat_map { |category_id| Category.subcategory_ids(category_id) }
+    end
+
+    @scope = @scope.joins(:category).where("categories.id IN (?)", category_ids)
+  end
+
   def filter_tags(tag_names:, match_all: true, exclude: false)
     return @scope if !SiteSetting.tagging_enabled?
     return @scope if tag_names.blank?

spec/lib/topics_filter_spec.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe TopicsFilter do
+  fab!(:user) { Fabricate(:user) }
   fab!(:admin) { Fabricate(:admin) }
   fab!(:group) { Fabricate(:group) }
 
@@ -23,6 +24,143 @@ RSpec.describe TopicsFilter do
       end
     end
 
+    describe "when filtering by categories" do
+      fab!(:category) { Fabricate(:category, name: "category") }
+
+      fab!(:category_subcategory) do
+        Fabricate(:category, parent_category: category, name: "category subcategory")
+      end
+
+      fab!(:category2) { Fabricate(:category, name: "category2") }
+
+      fab!(:category2_subcategory) do
+        Fabricate(:category, parent_category: category2, name: "category2 subcategory")
+      end
+
+      fab!(:private_category) do
+        Fabricate(:private_category, group: group, slug: "private-category")
+      end
+
+      fab!(:topic_in_category) { Fabricate(:topic, category: category) }
+      fab!(:topic_in_category_subcategory) { Fabricate(:topic, category: category_subcategory) }
+      fab!(:topic_in_category2) { Fabricate(:topic, category: category2) }
+      fab!(:topic_in_category2_subcategory) { Fabricate(:topic, category: category2_subcategory) }
+      fab!(:topic_in_private_category) { Fabricate(:topic, category: private_category) }
+
+      describe "when query string is `-category:category`" do
+        it "ignores the filter because the prefix is invalid" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("-category:category")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_in_category.id,
+            topic_in_category_subcategory.id,
+            topic_in_category2.id,
+            topic_in_category2_subcategory.id,
+            topic_in_private_category.id,
+          )
+        end
+      end
+
+      describe "when query string is `category:private-category`" do
+        it "should not return any topics when user does not have access to specified category" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("category:private-category")
+              .pluck(:id),
+          ).to eq([])
+        end
+
+        it "should return topics from specified category when user has access to specified category" do
+          group.add(user)
+
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new(user))
+              .filter_from_query_string("category:private-category")
+              .pluck(:id),
+          ).to contain_exactly(topic_in_private_category.id)
+        end
+      end
+
+      describe "when query string is `category:category`" do
+        it "should return topics from specified category and its subcategories" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("category:category")
+              .pluck(:id),
+          ).to contain_exactly(topic_in_category.id, topic_in_category_subcategory.id)
+        end
+
+        it "should return topics from specified category, its subcategories and sub-subcategories" do
+          SiteSetting.max_category_nesting = 3
+
+          category_subcategory_subcategory =
+            Fabricate(
+              :category,
+              parent_category: category_subcategory,
+              name: "category subcategory subcategory",
+            )
+
+          topic_in_category_subcategory_subcategory =
+            Fabricate(:topic, category: category_subcategory_subcategory)
+
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("category:category")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_in_category.id,
+            topic_in_category_subcategory.id,
+            topic_in_category_subcategory_subcategory.id,
+          )
+        end
+      end
+
+      describe "when query string is `category:category,category2`" do
+        it "should return topics from any of the specified categories and its subcategories" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("category:category,category2")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_in_category.id,
+            topic_in_category_subcategory.id,
+            topic_in_category2.id,
+            topic_in_category2_subcategory.id,
+          )
+        end
+      end
+
+      describe "when query string is `=category:category`" do
+        it "should not return topics from subcategories`" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("=category:category")
+              .pluck(:id),
+          ).to contain_exactly(topic_in_category.id)
+        end
+      end
+
+      describe "when query string is `=category:category,category2`" do
+        it "should not return topics from subcategories" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("=category:category,category2")
+              .pluck(:id),
+          ).to contain_exactly(topic_in_category.id, topic_in_category2.id)
+        end
+      end
+    end
+
     describe "when filtering by status" do
       fab!(:topic) { Fabricate(:topic) }
       fab!(:closed_topic) { Fabricate(:topic, closed: true) }