A function to generate keyset paginated queries for mongodb

generate-pagination-query.js

function generatePaginationQuery(query, sort, nextKey) {
  const sortField = sort == null ? null : sort[0];

  function nextKeyFn(items) {
    if (items.length === 0) {
      return null;
    }

    const item = items[items.length - 1];

    if (sortField == null) {
      return { _id: item._id };
    }

    return { _id: item._id, [sortField]: item[sortField] };
  }

  if (nextKey == null) {
    return { paginatedQuery: query, nextKeyFn };
  }

  let paginatedQuery = query;

  if (sort == null) {
    paginatedQuery._id = { $gt: nextKey._id };
    return { paginatedQuery, nextKey };
  }

  const sortOperator = sort[1] === 1 ? "$gt" : "$lt";

  const paginationQuery = [
    { [sortField]: { [sortOperator]: nextKey[sortField] } },
    {
      $and: [
        { [sortField]: nextKey[sortField] },
        { _id: { [sortOperator]: nextKey._id } }
      ]
    }
  ];

  if (paginatedQuery.$or == null) {
    paginatedQuery.$or = paginationQuery;
  } else {
    paginatedQuery = { $and: [query, { $or: paginationQuery }] };
  }

  return { paginatedQuery, nextKeyFn };
}

hey ... i think this line has a bug:

return { query, nextKeyFn };

i do not think it works for the first page because it needs to returned a named variable called paginatedQuery while here u r returning query ... so i think this needs to be:

return { paginatedQuery: query, nextKeyFn };

@rares-lupascu you are totally right

Imagine you have a column with duplicated data and you want to sort it. In this case this method does not work correctly. Do you have any idea? @mostafa-hz

nextKeyFn helps to advance forward. How do you save the previousKey to move backward.

@r-jalali this is where you should use a tie-breaker.
read the article here: https://medium.com/swlh/mongodb-pagination-fast-consistent-ece2a97070f3

@vijay122 I answered the question here: https://rawacc2031.medium.com/good-article-i-understood-how-to-fetch-next-page-one-after-other-what-if-i-want-previous-page-b642cfdcdd0f

That works 👏🏻