My understanding is that if you had money from before the marriage and you mix it with those earned during the marriage, unless you have documentation that you kept your money separate, your 'separate' money becomes community property?

I know there are a bunch of unfair family codes, but how is this one even fair? there's the provision for you to reimburse the community if you use community property to pay separate debt. why isn't there a provision for your separate property to be reimbursed if you used it on community expenses? b/c obviously, if you spent your separate property during the marriage, it was used on community property... right?