The Reds poor form away from home continued on the south coast as they were edged out in an entertaining game against Steve Cotterill's resurgent home side.
Paul Anderson had levelled Sonko's opener but Liam Lawrence netted a second half winner to leave Billy Davies's side stuck in the bottom half of the Championship.
And the Scot told his after-match press call that his side had some lessons to learn from their opponents.
"When you look at the game, what we need to learn here is that maybe we should look to take a leaf out of their (Portsmouth's) book and the way in which they play.
"We want to play the Forest way and we did play some wonderful football during periods of the game. But we lost soft goals."