custom_validator_spec.rb

RSpec.describe MyModel do
  subject { MyModel.new }

  %w(attribute1 attribute2 attribute3).each do |attribute|
    it "adds an error when #{attribute} is not divisible by 0.5" do
      subject.send("#{attribute}=", 1.3)

      subject.validate

      expect(subject.errors.messages[attribute]).to eq 'is not divisible by 0.5'
    end

    it "does not add an error when #{attribute} is divisible by 0.5" do
      subject.send("#{attribute}=", 1.5)

      subject.validate

      expect(subject.errors.messages[attribute]).to have(0).items
    end
  end
end