title: Rails Validation: Limit input to specific values
body: <p>I'm looking for "the Rails Way" to write a validation that limits acceptable input values to a predetermined list.</p>
  
  <p>In my case, I want to only accept the values "-5", "-2", "+2", "+5", and nil. However, I think this is best as a general question: how do you predefine a list of acceptable entry values in a Rails model?</p>
  
  <p>Thanks!</p>