DEV: Properly support composing multiple category filters on `/filter` (#20953)

Before this commit, composing multiple category filters with a query such as category:category1 and category:category2 would not return any results. This is because we were filtering for topics that belonged to both category1 and category2, which is impossible since a topic can only belong to a single category.

With this commit, specifying a query like category:category1 category:category2 will now translate to filtering for topics that belong to either the category1 or category2 category.

lib/topics_filter.rb

@@ -8,6 +8,7 @@ class TopicsFilter
 
   def filter_from_query_string(query_string)
     return @scope if query_string.blank?
+    category_or_clause = false
 
     query_string.scan(
       /(?<key_prefix>[-=])?(?<key>\w+):(?<value>[^\s]+)/,
@@ -45,7 +46,10 @@ class TopicsFilter
             filter_categories(
               category_slugs: category_slugs.split(delimiter),
               exclude_subcategories: key_prefix.presence,
+              or_clause: category_or_clause,
             )
+
+          category_or_clause = true
         end
       end
     end
@@ -80,7 +84,7 @@ class TopicsFilter
 
   private
 
-  def filter_categories(category_slugs:, exclude_subcategories: false)
+  def filter_categories(category_slugs:, exclude_subcategories: false, or_clause: false)
     category_ids = Category.ids_from_slugs(category_slugs)
 
     category_ids =
@@ -96,7 +100,11 @@ class TopicsFilter
       category_ids = category_ids.flat_map { |category_id| Category.subcategory_ids(category_id) }
     end
 
-    @scope = @scope.joins(:category).where("categories.id IN (?)", category_ids)
+    if or_clause
+      @scope.or(Topic.where("categories.id IN (?)", category_ids))
+    else
+      @scope.joins(:category).where("categories.id IN (?)", category_ids)
+    end
   end
 
   def filter_tags(tag_names:, match_all: true, exclude: false)

spec/lib/topics_filter_spec.rb

@@ -138,6 +138,37 @@ RSpec.describe TopicsFilter do
         end
       end
 
+      describe "when query string is `category: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 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 =category:category2`" do
+        it "should return topics and subcategory topics from category but only topics from category2" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("category:category =category:category2")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_in_category.id,
+            topic_in_category_subcategory.id,
+            topic_in_category2.id,
+          )
+        end
+      end
+
       describe "when query string is `=category:category`" do
         it "should not return topics from subcategories`" do
           expect(