separate business layer from infrastructure layer before service.js

service.js

export default (db) => {
  return {
    async checkIn(teamID) {
      const disqualifiedPlayers = await db.collection('playerDisqualifications').find({
        teamID
      })
        .toArray();
      if (disqualifiedPlayers.length > 0) {
        throw new Error('cannot check-in with disqualified players');
      }
      return db.collection('checkIns').insert({
        teamID,
        checkedInAt: new Date()
      });
    }
  };
};