Obtaining the index of items that are returned by using the limit and sorting by descending CreateAt

Question

Obtaining the index of items that are returned by using the limit and sorting by descending CreateAt

My first experience with Parse has been great so far. I've successfully worked on reading and writing data.

Now I am faced with a seemingly simple task, but in the Parse context, it feels like a challenge to implement and my usual search skills are not helping me much.

I am trying to retrieve items using code snippet below:

var query = new Parse.Query(CommentClass);
query.limit(2);
query.descending('createdAt');
query.find({
    success: function(object) {
        callback({
            comments: object
        });
    },
    error: function(object, error) {
        console.log('there was an error');
    }
});

The problem is, I need information about the indexes of the returned items compared to the total list. For example, if there are 5 items in total, this code will return the last 2 created items. However, there is no direct way to know the position of these items in the complete list without making another request.

javascript express parse-platform

Answer 1

Answer №1

To implement auto-incrementing sequences in Parse, one approach is to maintain a global index within a separate object. When adding a new comment, you would retrieve this global index, increment it, and assign it to the comment. However, managing deletions can complicate this process. Consider the following example without comment deletions:

SequenceForComment.js

// Define a class named SequenceForComment with a single row containing an integer property 'sequence'.
// Create this row using the Parse dashboard initially.

var SequenceForComment = Parse.Object.extend("SequenceForComment");

function getSequenceForComment(callback) {
  var query = new Parse.Query(SequenceForComment);
  return query.first().then(function (object) {
    // Atomic increment operation
    object.increment('sequence');
    return object.save();
  }).then(function (object) {
    callback(object.get('sequence'));
  }, function (error) {
    console.log(error);
    callback(undefined);
  });
}

module.exports = {
  getSequenceForComment: getSequenceForComment
};

main.js

var SequenceModule = require("cloud/SequenceForComment.js");

Parse.Cloud.beforeSave("Comment", function(request, response) {
  var comment = request.object;

  if (comment.isNew()) {
    SequenceModule.getSequenceForComment(function(sequence) {
      if (sequence) {
        comment.set("sequence", sequence);
        response.success();
      } else {
        response.error('Could not obtain a sequence.');
      }
    });
  } else {
    response.success();
  }
});

Answer 2

To implement auto-incrementing sequences in Parse, one approach is to maintain a global index within a separate object. When adding a new comment, you would retrieve this global index, increment it, and assign it to the comment. However, managing deletions can complicate this process. Consider the following example without comment deletions:

SequenceForComment.js

// Define a class named SequenceForComment with a single row containing an integer property 'sequence'.
// Create this row using the Parse dashboard initially.

var SequenceForComment = Parse.Object.extend("SequenceForComment");

function getSequenceForComment(callback) {
  var query = new Parse.Query(SequenceForComment);
  return query.first().then(function (object) {
    // Atomic increment operation
    object.increment('sequence');
    return object.save();
  }).then(function (object) {
    callback(object.get('sequence'));
  }, function (error) {
    console.log(error);
    callback(undefined);
  });
}

module.exports = {
  getSequenceForComment: getSequenceForComment
};

main.js

var SequenceModule = require("cloud/SequenceForComment.js");

Parse.Cloud.beforeSave("Comment", function(request, response) {
  var comment = request.object;

  if (comment.isNew()) {
    SequenceModule.getSequenceForComment(function(sequence) {
      if (sequence) {
        comment.set("sequence", sequence);
        response.success();
      } else {
        response.error('Could not obtain a sequence.');
      }
    });
  } else {
    response.success();
  }
});

Obtaining the index of items that are returned by using the limit and sorting by descending CreateAt

Answer №1

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