How to use associations in model validation?

Having a model1 with belongs_to :model2 association. model2 has an attribute called duration with an integer. model1 has an attribute called start_day. In this model, I want to validate that start_day is less than or equal to duration of model2.开发者_运维技巧

I added the following line to model1:

validates_numericality_of :start_day, :less_than_or_equal_to => :model2.duration

Rails fires a NoMethodError (undefined method `duration' for :model2:Symbol)

model2 has an attribute duration...

Any idea what I am missing? Thanks in advance...

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The less_than_or_equal_to option needs a numerical value, a symbol that names an instance method, or a Proc to be executed at validation time. You probably want the third option:

validates_numericality_of :start_day,
    :less_than_or_equal_to => Proc.new { |model1| model1.model2.duration }

Or if you have a method in model1 called model2_duration you could shorten it to this:

validates_numericality_of :start_day, :less_than_or_equal_to => :model2_duration