Search for issues with multiple assignees

app/finders/issuable_finder.rb

@@ -309,18 +309,6 @@ class IssuableFinder
     params[:sort] ? items.sort(params[:sort], excluded_labels: label_names) : items.reorder(id: :desc)
   end
 
-  def by_assignee(items)
-    if assignee
-      items = items.where(assignee_id: assignee.id)
-    elsif no_assignee?
-      items = items.where(assignee_id: nil)
-    elsif assignee_id? || assignee_username? # assignee not found
-      items = items.none
-    end
-
-    items
-  end
-
   def by_author(items)
     if author
       items = items.where(author_id: author.id)

app/finders/issues_finder.rb

@@ -95,7 +95,13 @@ class IssuesFinder < IssuableFinder
   end
 
   def by_assignee(items)
-    if assignee
+    if assignees.any?
+      assignees.each do |assignee|
+        items = items.assigned_to(assignee)
+      end
+
+      items
+    elsif assignee && assignees.empty?
       items.assigned_to(assignee)
     elsif no_assignee?
       items.unassigned
@@ -106,6 +112,19 @@ class IssuesFinder < IssuableFinder
     end
   end
 
+  def assignees
+    return @assignees if defined?(@assignees)
+
+    @assignees =
+      if params[:assignee_ids]
+        User.where(id: params[:assignee_ids])
+      elsif params[:assignee_usernames]
+        User.where(username: params[:assignee_usernames])
+      else
+        []
+      end
+  end
+
   def item_project_ids(items)
     items&.reorder(nil)&.select(:project_id)
   end

app/finders/merge_requests_finder.rb

@@ -24,6 +24,18 @@ class MergeRequestsFinder < IssuableFinder
 
   private
 
+  def by_assignee(items)
+    if assignee
+      items = items.where(assignee_id: assignee.id)
+    elsif no_assignee?
+      items = items.where(assignee_id: nil)
+    elsif assignee_id? || assignee_username? # assignee not found
+      items = items.none
+    end
+
+    items
+  end
+
   def item_project_ids(items)
     items&.reorder(nil)&.select(:target_project_id)
   end

spec/finders/issues_finder_spec.rb

@@ -59,6 +59,36 @@ describe IssuesFinder do
         end
       end
 
+      context 'filtering by assignee IDs' do
+        set(:user3) { create(:user) }
+        let(:params) { { assignee_ids: [user2.id, user3.id] } }
+
+        before do
+          project2.team << [user3, :developer]
+
+          issue3.assignees = [user2, user3]
+        end
+
+        it 'returns issues assigned to those users' do
+          expect(issues).to contain_exactly(issue3)
+        end
+      end
+
+      context 'filtering by assignee usernames' do
+        set(:user3) { create(:user) }
+        let(:params) { { assignee_usernames: [user2.username, user3.username] } }
+
+        before do
+          project2.team << [user3, :developer]
+
+          issue3.assignees = [user2, user3]
+        end
+
+        it 'returns issues assigned to those users' do
+          expect(issues).to contain_exactly(issue3)
+        end
+      end
+
       context 'filtering by author ID' do
         let(:params) { { author_id: user2.id } }