parent has_many :sons, son belongs_to :parent, 根据sons的数量排序
@parents = Parent.all.sort_by{ |u| u.sons.count}
or
@parents = Parent.all.sort{ |a,b| a.sons.count <=> b.sons.count}
如果要取反
@parents = Parent.all.sort_by{ |u| -u.sons.count}
or
@parents = Parent.all.sort{ |a,b| b.sons.count <=> a.sons.count}