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Default Attributes on Active Record Associations in Rails

I worked up a quick Active Record association extention that allows you to define default attributes on new records created through the association:


class User < ActiveRecord::Base
  has_many :events, :extend => AssociationDefaultAttributes do
    def default_attributes
      { :venue => proxy_owner.default_venue }
    end
  end
end

module AssociationDefaultAttributes
  def create(attrs, &block)
    super(default_attributes.merge(attrs), &block)
  end

  def build(attrs={}, &block)
    super(default_attributes.merge(attrs), &block)
  end
end

What this is doing is extending the association proxy object with a module that overrides the build and create methods such that they use the default values returned by the default_attributes method. Much neater than setting them elsewhere, no?

I love this sort of meta-programming in Ruby, it’s so much nice than the way we used to do it in the bad old days using alias_method_chain.

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