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class Activity < ActiveRecord::Base
has_and_belongs_to_many :balance_sheets
end
和
class BalanceSheet < ActiveRecord::Base
has_and_belongs_to_many :activities
accepts_nested_attributes_for :activities
end
我,如果我有加载balance_sheet对象上以前的活动执行UPDATE发送"balance_sheet"=>{"activity_ids"=>["10", "20"]}
,活动集合被替换。 我不想覆盖活动的旧值,我想添加新的。我怎样才能做到这一点?