udpated a few things based on MR feedback. Also added model spec

app/finders/issuable_finder.rb

@@ -270,8 +270,7 @@ class IssuableFinder
       if filter_by_no_label?
         items = items.without_label
       else
-        items = items.with_label(label_names.flatten)
-
+        items = items.with_label(label_names)
         if projects
           items = items.where(labels: { project_id: projects })
         end
@@ -282,7 +281,7 @@ class IssuableFinder
   end
 
   def label_names
-    params[:label_name].split(',')
+    params[:label_name].is_a?(String) ? params[:label_name].split(',') : params[:label_name]
   end
 
   def current_user_related?

app/helpers/milestones_helper.rb

@@ -32,7 +32,7 @@ module MilestonesHelper
   end
 
   def milestone_issues_by_label_count(milestone, label, state:)
-    milestone.issues.with_label([label.title]).send(state).size
+    milestone.issues.with_label(label.title).send(state).size
   end
 
   def milestone_progress_bar(milestone)

app/models/concerns/issuable.rb

@@ -37,7 +37,6 @@ module Issuable
     scope :closed, -> { with_state(:closed) }
     scope :order_milestone_due_desc, -> { joins(:milestone).reorder('milestones.due_date DESC, milestones.id DESC') }
     scope :order_milestone_due_asc, -> { joins(:milestone).reorder('milestones.due_date ASC, milestones.id ASC') }
-    scope :with_label, ->(title) { joins(:labels).where(labels: { title: title }).group('issues.id').having("count(distinct labels.title) = #{title.count}") }
     scope :without_label, -> { joins("LEFT OUTER JOIN label_links ON label_links.target_type = '#{name}' AND label_links.target_id = #{table_name}.id").where(label_links: { id: nil }) }
 
     scope :join_project, -> { joins(:project) }
@@ -122,6 +121,14 @@ module Issuable
 
       joins(join_clause).group(issuable_table[:id]).reorder("COUNT(notes.id) DESC")
     end
+
+    def with_label(title)
+      if title.is_a?(Array) && title.count > 1
+        joins(:labels).where(labels: { title: title }).group('issues.id').having("count(distinct labels.title) = #{title.count}")
+      else
+        joins(:labels).where(labels: { title: title })
+      end
+    end
   end
 
   def today?

spec/models/concerns/issuable_spec.rb

@@ -212,4 +212,28 @@ describe Issue, "Issuable" do
       expect(issue.downvotes).to eq(1)
     end
   end
-end
+
+  describe ".with_label" do
+    let(:example_label) { 'test1' }
+    let(:example_labels) { ['test1', 'test2'] }
+
+    it 'finds issue with 1 label' do
+      setup_labels([example_label])
+
+      expect(Issue.with_label(example_label).count).to eq(1)
+    end
+
+    it 'finds issue with 2 labels' do
+      setup_labels(example_labels)
+
+      expect(Issue.with_label(example_labels).to_a.count).to eq(1)
+    end
+
+    def setup_labels(label_names)
+      labels = label_names.map do |label|
+        create(:label, project: issue.project, title: label)
+      end
+      issue.labels << labels
+    end
+  end
+end
\ No newline at end of file