Do the following two lines of code behave in exactly the same way despite slightly different implementations
values.map{ |k,v| __send__('%s=' % k.to_s, v) }
values.map{ |k,v| __send__("#{k.to_s}=", v) }
The second line would be a more common ruby idiom so I was wondering why the other method was used when this was in the Rails Core which I would expect to use idiomatic ruby.
They are not absolutely identical. For instance, the first example will call String#%, so if that method is redefined for some strange reason, you might get a different result. With the standard definition of String#%, the strings computed will be the same, so both expressions will have the same result.
BTW, there's no need for to_s in this example, and assuming send has not been redefined (and is thus equivalent to __send__):
values.map{ |k,v| send("#{k}=", v) }
__send__ as this is within a constructor where people could pass in a key of send.