The chords above that @GanglyCloth posted show the A natural minor scale and the A harmonic minor scale. There is no hint of the A melodic minor scale (which would have an F# in it).
In general, the notes of the natural minor are what are used as the functional notes and harmony for songs in a minor key - particularly pop songs. The adjustment of adding notes outside the key signature (in the case of the example, the G#) would only happen when the chord/harmony called for it.
Keys are helpful in the macro view, and they are helpful in talking about chord progressions and relationships. But in the trenches of playing a song, it is much more accurate and helpful to play to the specific harmony at any given moment.
The melodic minor is a slippery fish and has all kinds of influence and use in jazz and extended harmonies - but it is very very rarely found in any pop music.
Harmonic minor is rarely used as a scale - it is named as such because the only note that changes (compared to the natural minor) is changed at very specific times only in the music and only to adjust the harmony of the V chord to make it lead towards the minor tonic chord.
Wow. I need a Twitter-like word limit on this.
All I mean to say @T_dub is that you can only use Harmonic Minor or Melodic Minor when the harmony of the music specifically brings in those altered notes.
Damn.
Hope something up there makes sense.