Association merging and don't forget to .reload

Recently I had to develop a merge function, student A (master) needs to merge with student B (slave), slave have a lot of associations to be merged, in particular comments, so here is the merge function

def merge_student(master, slave)
  slave.comments.each do |comment|
    comment.student = master
    comment.save!
  end

  master.save!
  slave.reload # <--- at first I forgot to add that line
  slave.destroy
end

At first I didn’t add the slave.reload and thought that all the comments would be successfully migrated, but actually the slave.comments were all destroyed.

Because when we do slave.destroy, slave still has its original comments associations, so I needed to make sure I call slave.reload before doing the slave.destroy.