Type cast ActiveModel attributes on write instead of read?

rubyonrails-talk
1

I’m building a project using a few ActiveModel APIs including ActiveModel::Attributes. I have various objects that associate to each other via attributes, with custom ActiveModel types that support being cast from a small number of other objects (mainly String). Mostly, things are working great, but I noticed that I can assign invalid attributes without raising exceptions until reading that attribute. For example:

class PlainText
  def initialize(text:)
    @text = text
  end

  def as_json(*)
    {type: "plain_text", text: text}
  end
end

class Types::PlainText < ActiveModel::Type::Value
  def cast(value)
    case value
    when ::PlainText
      value
    when String
      ::PlainText.new(value)
    else
      raise ArgumentError, "Cannot implicitly cast `#{value}' to PlainText"
    end
  end
end

class Header
  attribute :text, Types::PlainText.new
end

header = Header.new
header.text = "Hello, world!"
# => "Hello, world!"

header.text
# => #<PlainText @text="Hello, world!">

header.text = :this_is_invalid
# => :this_is_invalid

header.text
# => ArgumentError: Cannot implicitly cast `:this_is_invalid' to PlainText

Is there any way to have casting fail on write? I’m worried that I’ll have to override the setters for every attribute like this, which defeats a large purpose of using ActiveModel, IMO.