What is a good architecture for storing game statistics, so that I can retrieve snapshots of it at various moments?

Say I have a game, and the user's statistics initially are:

 {
 hours_played = 0,
 games_played = 0,
 no_of_times_killed = 0,
 }

When the user purchases something for the first time from within the game, the stats are

 {
 hours_played = 10,
 games_played = 2,
 no_of_times_killed = 5,
 }

And when he purchases something for the second time, the stats are

 {
 hours_played = 20,
 games_played = 4,
 no_of_times_killed = 10,
 }

Let me name the events as "purchase1" and "purchase2"

How do I model my statistics, so that at any point in the future, I will be able to retrieve the snapshot of the statistics at the time when "purchase1" was fired. Similarly for "purchase2" and any other event I will add.

Hope that made sense..