Apply similar change to MRs API

app/finders/issues_finder.rb

@@ -135,6 +135,7 @@ class IssuesFinder < IssuableFinder
     current_user.blank?
   end
 
+  # rubocop: disable CodeReuse/ActiveRecord
   def by_assignee(items)
     if filter_by_no_assignee?
       items.unassigned

lib/api/issues.rb

@@ -42,7 +42,7 @@ module API
         optional :author_id, type: Integer, desc: 'Return issues which are authored by the user with the given ID'
         optional :assignee_id, types: [Integer, String],
                                values: -> (v) {
-                                 v.is_a?(Integer) or [IssuableFinder::FILTER_NONE, IssuableFinder::FILTER_ANY].include?(v)
+                                 v.is_a?(Integer) || [IssuableFinder::FILTER_NONE, IssuableFinder::FILTER_ANY].include?(v)
                                },
                                desc: 'Return issues which are assigned to the user with the given ID'
         optional :scope, type: String, values: %w[created-by-me assigned-to-me created_by_me assigned_to_me all],

lib/api/merge_requests.rb

@@ -89,7 +89,11 @@ module API
         optional :updated_before, type: DateTime, desc: 'Return merge requests updated before the specified time'
         optional :view, type: String, values: %w[simple], desc: 'If simple, returns the `iid`, URL, title, description, and basic state of merge request'
         optional :author_id, type: Integer, desc: 'Return merge requests which are authored by the user with the given ID'
-        optional :assignee_id, type: Integer, desc: 'Return merge requests which are assigned to the user with the given ID'
+        optional :assignee_id, types: [Integer, String],
+                               values: -> (v) {
+                                 v.is_a?(Integer) || [IssuableFinder::FILTER_NONE, IssuableFinder::FILTER_ANY].include?(v)
+                               },
+                               desc: 'Return merge requests which are assigned to the user with the given ID'
         optional :scope, type: String, values: %w[created-by-me assigned-to-me created_by_me assigned_to_me all],
                          desc: 'Return merge requests for the given scope: `created_by_me`, `assigned_to_me` or `all`'
         optional :my_reaction_emoji, type: String, desc: 'Return issues reacted by the authenticated user by the given emoji'

spec/requests/api/issues_spec.rb

@@ -188,6 +188,9 @@ describe API::Issues do
       end
 
       it 'returns issues with any assignee' do
+        # This issue without assignee should not be returned
+        create(:issue, author: user2, project: project)
+
         get api('/issues', user), assignee_id: 'Any', scope: 'all'
 
         expect_paginated_array_response(size: 3)

spec/requests/api/merge_requests_spec.rb

@@ -143,6 +143,23 @@ describe API::MergeRequests do
         expect_response_ordered_exactly(merge_request3)
       end
 
+      it 'returns an array of merge requests with no assignee' do
+        merge_request3 = create(:merge_request, :simple, author: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+
+        get api('/merge_requests', user), assignee_id: 'None', scope: :all
+
+        expect_response_ordered_exactly(merge_request3)
+      end
+
+      it 'returns an array of merge requests with any assignee' do
+        # This MR with no assignee should not be returned
+        create(:merge_request, :simple, author: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+
+        get api('/merge_requests', user), assignee_id: 'Any', scope: :all
+
+        expect_response_contain_exactly(merge_request, merge_request2, merge_request_closed, merge_request_merged, merge_request_locked)
+      end
+
       it 'returns an array of merge requests assigned to me' do
         merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')