How can I retrieve the record before the current one in a Meteor JS collection?

I need assistance with comparing timestamps and userIds of messages in a collection. Specifically, I want to access the data of the previous message in relation to the current one when rendering each message in the collection. Any tips on how to achieve this would be greatly appreciated.

Below are my templates:

<template name="messageList">
  <ul>
    {{#each messages}}
      {{>messageItem}}
    {{/each}}
  </ul>
</template>

<template name="messageItem">
  <li>
    <p class="{{nameVisibility}}">{{userName}}</p>
    <p>{{body}}</p>
  </li>
</template>

This is from my helpers file:

Template.messageItem.helpers({
  nameVisibility: function() {

    //I am unsure how to query the previous message.
    previousMessage = Messages.findOne(...);
    if (this.userId != previousMessage.userId) {
      return false;
    else {
      return "hideName";
    }  
  }
});

Answer №1

I have a differing opinion from the previous response in this thread, as I believe it is not the most efficient method and requires unnecessary iteration over the entire Messages collection.

In my view, utilizing regular jQuery within the Template.messageItem.rendered callback would be the optimal approach. By locating the preceding element, utilizing UI.getElementData to retrieve its data, and subsequently either adding or removing the username appropriately, you can achieve the desired outcome. With Meteor 0.8.0+ (Blaze rendering engine), your modifications will persist. Here's an example implementation:

Template.messageItem.rendered = function() {
  // this.firstNode represents the <li> tag; .prev() selects its previous sibling
  prevMessage = $(this.firstNode).prev();
  prevData = UI.getElementData(prevMessage[0]); // ensure prevMessage[0] exists
  if (prevData.userName === this.data.userName)
    this.$(".username").remove(); // Ensure the ".username" class is on the prefix
}

Answer №2

Prepare the array being iterated through:

Template.messageList.helpers({
  messages: function() {
    var i, j, messages;
    messages = Messages.find({}).fetch(); // Adjust `{}` to fit your query
    for (i = 0, j = ret.length; i < j; i++) {
      if (messages[i-i] != null && messages[i].userId == messages[i-1].userId)
        messages[i].nameVisibility = "hideName";
    }
    return messages;
  }
});

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