JavaScript Meteor: How to return only one entry for each value from collection

I am currently working on a Meteor application that also has a chat functionality.

I want to have a list of all conversations that show the most recent message in each conversation. (Similar to Sample of a basic conversation overview)

Let's say I have a collection Messages with a variable conversationId. New messages get assigned a new conversationId and any replies will get assigned the conversationId of the first message.

To achieve this conversation overview, my question is, how do I return from my collection only the most recent entry for each conversationId?

This is where I am stuck: 

Template.tabsTwo.helpers({
  messages: function () {

    //to retrieve all messages from the logged in user, then retrieve the conversationIDs and drop duplicates
    var userMessages = Messages.find({senderId: Meteor.userId()}, {sort: {date_created: -1, subject: -1}});

    var userConversationIds = userMessages.map((function(a) {
      return a.conversationId;
    }));

    var uniqueConversationIDs = [];

    $.each(userConversationIds, function(i, el){
        if($.inArray(el, uniqueConversationIDs) === -1) uniqueConversationIDs.push(el);
    });

    return Messages.find({conversationId: {$in:uniqueConversationIDs}}, {sort: {date_created: -1}});
  }
});

This still gives me back all messages. I am asking myself right now if I can modify this query to make it work or if I need to approach this differently (e.g. do a loop and a .findOne query)?

(I have tried many things and searched for answers in the docs and SO but have troubles getting this right. Any help would be greatly appreciated.)

Answer:1

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