Ok, the old school answer, i.e.. (unschooled), is first off, DMAJ7, does not contain a flatted root(c) but the natural 7th scale tone C#, DF#AC#, which are all in the D major scale, Dmin7th, is DFAC, the C is the 7th in the Dmin7th chord, now here's an answer to the scale mystery, if you are playing in C Major, then Dm7th fits into your scale CDEFGABC, now start on the 2nd degree of that C scale, changing the root of the scale to D, play DEFGABCD, you are now playing in one of the minor modes, but I can't ever remember which one, maybe Dorian or Chick O Fillian, or sumptin like , does that help? or make sense? I hope it helps