Should I put this Invariant within a domain service or aggregate itself?

I'm working on a side project to learn and apply DDD within the "Daily
Deal' domain. In my purchasing context, i have an invariant where a user
can only purchase 'x' amount of deals per deal.

so it seems wasteful for my deal aggregate to load all purchases from all
users just to check and see how many times (if any) the user has purchased
this deal. I see two ways i could go about this.

1. Put this logic within a domain service which would allow a pre-condition
to already have been met when the Purchase method on the Deal aggregate is
invoked.

2. My repository implementation could always populate the purchases
collection of the deal for the purchasing user. hmm...not sure about this
one.

any guidance would be great!