Filter issues without an Assignee via the API

app/finders/issuable_finder.rb

@@ -236,16 +236,16 @@ class IssuableFinder
   # rubocop: enable CodeReuse/ActiveRecord
   def assignee_id?
-    params[:assignee_id].present? && params[:assignee_id] != NONE
+    params[:assignee_id].present? && params[:assignee_id].to_s != NONE
   end
   def assignee_username?
-    params[:assignee_username].present? && params[:assignee_username] != NONE
+    params[:assignee_username].present? && params[:assignee_username].to_s != NONE
   end
   def no_assignee?
     # Assignee_id takes precedence over assignee_username
-    params[:assignee_id] == NONE || params[:assignee_username] == NONE
+    params[:assignee_id].to_s == NONE || params[:assignee_username].to_s == NONE
   end
   # rubocop: disable CodeReuse/ActiveRecord

changelogs/unreleased/41205-fix-filtering-issues.yml

+---
+title: Filter issues without an Assignee via the API
+merge_request: 22009
+author: Eva Kadlecová
+type: fixed

spec/finders/issues_finder_spec.rb

@@ -56,6 +56,14 @@ describe IssuesFinder do
         end
       end
+      context 'filtering by no assignee' do
+        let(:params) { { assignee_id: 0 } }
+
+        it 'returns issues not assign to any assignee' do
+          expect(issues).to contain_exactly(issue4)
+        end
+      end
+
       context 'filtering by group_id' do
         let(:params) { { group_id: group.id } }

spec/requests/api/issues_spec.rb

@@ -168,6 +168,15 @@ describe API::Issues do
         expect(first_issue['id']).to eq(issue2.id)
       end
+      it 'returns issues with no assignee' do
+        issue2 = create(:issue, author: user2, project: project)
+
+        get api('/issues', user), assignee_id: 0, scope: 'all'
+
+        expect_paginated_array_response(size: 1)
+        expect(first_issue['id']).to eq(issue2.id)
+      end
+
       it 'returns issues reacted by the authenticated user by the given emoji' do
         issue2 = create(:issue, project: project, author: user, assignees: [user])
         award_emoji = create(:award_emoji, awardable: issue2, user: user2, name: 'star')