[Multiple Issue Assignee] Fix issue filters

app/controllers/concerns/issuable_collections.rb

@@ -40,7 +40,7 @@ module IssuableCollections
   end
 
   def issues_collection
-    issues_finder.execute.preload(:project, :author, :assignees, :labels, :milestone, project: :namespace)
+    issues_finder.execute.preload(:project, :author, :labels, :milestone, project: :namespace)
   end
 
   def merge_requests_collection

app/finders/issues_finder.rb

@@ -26,12 +26,26 @@ class IssuesFinder < IssuableFinder
     IssuesFinder.not_restricted_by_confidentiality(current_user)
   end
 
+  def by_assignee(items)
+    if assignee
+      items = items.where("issue_assignees.user_id = ?", assignee.id)
+    elsif no_assignee?
+      items = items.where("issue_assignees.user_id is NULL")
+    elsif assignee_id? || assignee_username? # assignee not found
+      items = items.none
+    end
+
+    items
+  end
+
   def self.not_restricted_by_confidentiality(user)
-    return Issue.where('issues.confidential IS NULL OR issues.confidential IS FALSE') if user.blank?
+    issues = Issue.with_assignees
+
+    return issues.where('issues.confidential IS NULL OR issues.confidential IS FALSE') if user.blank?
 
-    return Issue.all if user.admin_or_auditor?
+    return issues.all if user.admin_or_auditor?
 
-    Issue.with_assignees.where('
+    issues.where('
       issues.confidential IS NULL
       OR issues.confidential IS FALSE
       OR (issues.confidential = TRUE