Publish documents in a collection to a meteor client depending on the existence of a specific document in another collection (publish-with-relations)

Question

I have two collections

1. Offers (relevant fields: _id)
2. ShareRelations (relevant fields: receiverId and offerId)

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Answer 1

You can use the reactive-publish package (I am one of authors):

Meteor.publish('offersShared', function () {
  // check if the user is logged in
  if (this.userId) {
    this.autorun(function (computation) {
      // initialize helper array
      var visibleOffers = [];
      // initialize all shareRelations which the actual user is the receiver
      var shareRelations = ShareRelations.find({receiverId: this.userId}, {fields: {offerId: 1}});
      // loop trough all shareRelations and push the offerId to the array if the value isn't in the array actually
      shareRelations.forEach(function (shareRelation) {
        if (visibleOffers.indexOf(shareRelation.offerId) === -1) {
          visibleOffers.push(shareRelation.offerId);
        }
      });
      // return offers which contain the _id in the array visibleOffers
      return Offers.find({_id:  { $in: visibleOffers } });
    });
  } else {
    // return no offers if the user is not logged in
    return Offers.find(null);
  }
});

You can simply wrap your existing non-reactive code into an autorun and it will start to work. Just be careful to be precise which fields you query on because if you query on all fields then autorun will be rerun on any field change of ShareRelations, not just offerId.