locks.diff

  ActiveRecord::Base.transaction do
-   # doing delete ... where task_id in (...) and subtask_id is null (inequality test on secondary key), 
-   # locks secondary indexes and primary key index
-   AssignedUser.where(task_id: task.id, subtask_id: nil).delete_all
+   assigned_user_ids_to_clean = AssignedUser.where(task_id: task.id, subtask_id: nil).pluck(:id)
+   # doing delete ... where id in (...) (equality test on primary key), locks only primary key index
+   AssignedUser.where(id: assigned_user_ids_to_clean).delete_all
    task.assigned_user_ids = assigned_user_ids # inserts multiple records
  end