ED503 ED503 - 6 months ago 35
Ruby Question

Calling method_missing for undefined attributes in Rails after using "update" method

Is there a short way for

obj.update(attr1: "something")
method to call
method_missing
instead of raising the error
ActiveModel::UnknownAttributeError (unknown attribute 'attr1' for Obj.)
?

I am thinking about simply rescuing it and then mimicking/calling the method_missing, but this way feels too bulky.

Answer

From the source code (Rails 4.2) it seems that ActiveRecord::UnknownAttributeError is raised when the model instance does not respond_to? to the given attribute setter. So what you have to do is define a method_missing as well as respond_to_missing? in the model for all your dynamic attributes. This is actually what you always should do when using method_missing anyway:

class Model < ActiveRecord::Base

  DYNAMIC_ATTRIBUTES = [:attr1, :attr2]

  def method_missing(method_name, *arguments, &block)
    if DYNAMIC_ATTRIBUTES.map { |attr| "#{attr}=" }.include?(method_name.to_s)
      puts "custom code for #{method_name} #{arguments.first.inspect}"
    else
      super
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    DYNAMIC_ATTRIBUTES.map { |attr| "#{attr}=" }.include?(method_name.to_s) || super
  end

end

Test in Rails console:

Model.first.update(attr1: 'aa', attr2: 'bb')
# => custom code for attr1= "aa"
# => custom code for attr2= "bb"

Model.first.update(attr1: 'aa', attr2: 'bb', attr3: 'cc')
# => ActiveRecord::UnknownAttributeError: unknown attribute 'attr3' for Model.
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