Fix issuable assignee update bug when previous assignee is null

app/models/concerns/issuable.rb

@@ -92,9 +92,10 @@ module Issuable
     after_save :record_metrics
 
     def update_assignee_cache_counts
-      # make sure we flush the cache for both the old *and* new assignee
-      User.find(assignee_id_was).update_cache_counts if assignee_id_was
-      assignee.update_cache_counts if assignee
+      # make sure we flush the cache for both the old *and* new assignees(if they exist)
+      previous_assignee = User.find_by_id(assignee_id_was)
+      previous_assignee.try(:update_cache_counts)
+      assignee.try(:update_cache_counts)
     end
 
     # We want to use optimistic lock for cases when only title or description are involved

changelogs/unreleased/issue_22664.yml

+---
+title: Fix issuable assignee update bug when previous assignee is null
+merge_request:
+author:

spec/models/concerns/issuable_spec.rb

@@ -44,6 +44,26 @@ describe Issue, "Issuable" do
     it { expect(described_class).to respond_to(:assigned) }
   end
 
+  describe "after_save" do
+    describe "#update_cache_counts" do
+      context "when previous assignee exists" do
+        it "user updates cache counts" do
+          expect(user).to receive(:update_cache_counts)
+
+          issue.update(assignee: user)
+        end
+      end
+
+      context "when previous assignee does not exist" do
+        it "does not raise error" do
+          issue.update(assignee_id: "")
+
+          expect { issue.update(assignee_id: user) }.not_to raise_error
+        end
+      end
+    end
+  end
+
   describe ".search" do
     let!(:searchable_issue) { create(:issue, title: "Searchable issue") }