Implement fulfillment of CertainInvites

[monitz87]

While implementing I ran into some questions:

• What do we do when a member with a fulfilled referral leaves the guild? I think we should mark that referral somehow (without it losing its fulfilled status) so that the scoring algorithm can take that into account if needed. The question that arises is what would happen if I invite someone, he spends some time in the guild, leaves, and then after some more time accepts an invite from someone else. If the time periods in between are sufficiently long, it shouldn't be considered cheating. I think the model should reflect that somehow.

• Should we cache part of our database in the service? Currently I am fetching a referral for each member for each guild every X seconds (that's a lot of queries to the database on each iteration). If we do, we have to figure out a caching strategy, because it's not feasible to cache our whole referral collection into memory.

[RafaelVidaurre]

@monitz87

• I see, I'm glad you considered this case, I definitely missed it. My approach would be this:
• fulfilled is never changed after set to true
• I agree that we should allow the scores to filter out members who left. We have two options here:
  1. Add a boolean (active for example) which we make the service try to maintain (we already do something extremely similary for the fulfilled field).
  2. Assuming we have a cronjob that calculates scores (we wont for now) we can just query all members when the score is gonna be calculated and subtract absent ones there
• I feel like the edge-case of a member joining again after six months is taking care of too much, even in a situation like that we could still argue that the user came back since they learned about the guild thanks to the first time they were invited. I think though, that the fact that some member can join again and not add to the score of a second inviter can definitely cause confusion. I'd suggest we allow suers to configure a channel where the bot can announce when a new user joined. There we could say who invited them, and if they are joining again we simply say something like Narzerus has returned to the server after 3 months! He was first invited by Monitz87

[RafaelVidaurre]

@monitz87 about caching. That's a tougher one, why do you have to query each referral for each member every x seconds by the way?

[monitz87]

We decided on not querying the database on an interval, but instead only doing it on startup and trusting that no events will be lost by discordjs.