If you want to generate a dropdown selector menu from a JSON using Ruby on Rails, you can do it with two lines of code (three including the JSON string).
In your controller, use the JSON parse method to convert the string into a hash.
myjsonstring = '{"1":"Test1","2":"Test2","3":"Test3"}' @myvar = JSON.parse(myjsonstring)
In your view,
<%= f.collection_select myfieldname, @myjsonstring, :mykey, :myvalue %>
The end result will look like the following with options values set to 1, 2, and 3.