It looks like the error is probably happening inside the description function, because you're not setting the id correctly.
Try changing your code to the following:
team_a_id = FactoryGirl.create(:team, name: "Team A").id
team_b_id = FactoryGirl.create(:team, name: "Team B").id
game = FactoryGirl.create(:game, team_a_id: team_a_id, team_b_id: team_b_id)
This should set the id's in the game model.