DEV: Support `posters-(min|max):<count>` on `/filter` route (#21095)

This commit adds support for the `posters-min:<count>` and
`posters-max:<count>` filters for the topics filtering query language.
`posters-min:1` will filter for topics with at least a one poster while
`posters-max:3` will filter for topics with a maximum of 3 posters.

If the filter has an invalid value, i.e string that cannot be converted
into an integer, the filter will be ignored.

If either of each filter is specify multiple times, only the last
occurence of each filter will be taken into consideration.

lib/topics_filter.rb

@@ -41,9 +41,21 @@ class TopicsFilter
       when "in"
         filter_in(values: values)
       when "posts-min"
-        filter_by_number_of_posts(min: values.last)
+        min = values.last
+        break if !integer_string?(min)
+        filter_by_number_of_posts(min: min)
       when "posts-max"
-        filter_by_number_of_posts(max: values.last)
+        max = values.last
+        break if !integer_string?(max)
+        filter_by_number_of_posts(max: max)
+      when "posters-min"
+        min = values.last
+        break if !integer_string?(min)
+        filter_by_number_of_posters(min: min)
+      when "posters-max"
+        max = values.last
+        break if !integer_string?(max)
+        filter_by_number_of_posters(max: max)
       when "status"
         values.each { |status| @scope = filter_status(status: status) }
       when "tags"
@@ -81,13 +93,21 @@ class TopicsFilter
 
   private
 
-  def filter_by_number_of_posts(min: nil, max: nil)
+  def filter_by_topic_range(column_name:, min: nil, max: nil)
     { min => ">=", max => "<=" }.each do |value, operator|
-      next if !value || value !~ /^\d+$/
-      @scope = @scope.where("topics.posts_count #{operator} ?", value)
+      next if !value
+      @scope = @scope.where("topics.#{column_name} #{operator} ?", value)
     end
   end
 
+  def filter_by_number_of_posts(min: nil, max: nil)
+    filter_by_topic_range(column_name: "posts_count", min:, max:)
+  end
+
+  def filter_by_number_of_posters(min: nil, max: nil)
+    filter_by_topic_range(column_name: "participant_count", min:, max:)
+  end
+
   def filter_categories(values:)
     exclude_subcategories_category_slugs = []
     include_subcategories_category_slugs = []
@@ -332,4 +352,8 @@ class TopicsFilter
   def include_topics_with_any_tags(tag_ids)
     @scope = @scope.joins(:topic_tags).where("topic_tags.tag_id IN (?)", tag_ids).distinct(:id)
   end
+
+  def integer_string?(string)
+    string =~ /\A\d+\z/
+  end
 end

spec/lib/topics_filter_spec.rb

@@ -929,5 +929,100 @@ RSpec.describe TopicsFilter do
         end
       end
     end
+
+    describe "when filtering by number of posters in a topic" do
+      fab!(:topic_with_1_participant) { Fabricate(:topic, participant_count: 1) }
+      fab!(:topic_with_2_participants) { Fabricate(:topic, participant_count: 2) }
+      fab!(:topic_with_3_participants) { Fabricate(:topic, participant_count: 3) }
+
+      describe "when query string is `posters-min:1`" do
+        it "should only return topics with at least 1 participant" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-min:1")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_with_1_participant.id,
+            topic_with_2_participants.id,
+            topic_with_3_participants.id,
+          )
+        end
+      end
+
+      describe "when query string is `posters-min:3`" do
+        it "should only return topics with at least 3 participants" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-min:3")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_3_participants.id)
+        end
+      end
+
+      describe "when query string is `posters-max:1`" do
+        it "should only return topics with at most 1 participant" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-max:1")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_participant.id)
+        end
+      end
+
+      describe "when query string is `posters-max:3`" do
+        it "should only return topics with at most 3 participants" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-max:3")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_with_1_participant.id,
+            topic_with_2_participants.id,
+            topic_with_3_participants.id,
+          )
+        end
+      end
+
+      describe "when query string is `posters-min:1 posters-max:2`" do
+        it "should only return topics with at least 1 participant and at most 2 participants" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-min:1 posters-max:2")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_1_participant.id, topic_with_2_participants.id)
+        end
+      end
+
+      describe "when query string is `posters-min:3 posters-min:2 posters-max:1 posters-max:3`" do
+        it "should only return topics with at least 2 participants and at most 3 participants as it ignores earlier filters which are duplicated" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-min:3 posters-min:2 posters-max:1 posters-max:3")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_2_participants.id, topic_with_3_participants.id)
+        end
+      end
+
+      describe "when query string is `posters-min:invalid posters-max:invalid`" do
+        it "should ignore the filters with invalid values" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("posters-min:invalid posters-max:invalid")
+              .pluck(:id),
+          ).to contain_exactly(
+            topic_with_1_participant.id,
+            topic_with_2_participants.id,
+            topic_with_3_participants.id,
+          )
+        end
+      end
+    end
   end
 end