2) MongoDB Query of Advanced Filtering, Sorting, Limit Field and Pagination with mongoose

Prerequisite

Kindly ensure you’ve gone through the earlier chapter of this article. mongodb-query-filter-sort-limit-pagination
You can also get the example data used in this article. tours.json

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exports.getAllTours = async (req, res) => {
  try {
    //BUILD QUERY
    // 1A) Filtering
    const queryObj = { ...req.query }
    const excludedFields = ['page', 'sort', 'limit', 'fields']
    excludedFields.forEach(el => delete queryObj[el])

    //1B) Advanced filtering
    let queryString = JSON.stringify(queryObj)
    queryString = queryString.replace(/\b(gte|gt|lte|lt)\b/g, match => `$${match}`)    
    let query = Tour.find(JSON.parse(queryString))

    // 2) Sorting
    if (req.query.sort) {
      const sortBy = req.query.sort.split(',').join(' ')
      query = query.sort(sortBy)
    } else {
      query = query.sort('-createdAt')
    }

    //3) Field Limiting
    // Select pattern  .select("firstParam secondParam"), it will only show the selected field, add minus sign for excluding (include everything except the given params)
    if (req.query.fields) {
      const fields = req.query.fields.split(',').join(' ')
      query = query.select(fields)
    } else {
      query = query.select('-__v')
    }

    // 4) Pagination
    // page=2&limit=10, 1-10 page 1, 11-20 page 2, 21-30 page 3
    const page = req.query.page * 1 || 1;
    const limit = req.query.limit * 1 || 100;
    const skip = (page - 1) * limit;

    query = query.skip(skip).limit(limit);

    //EXECUTE QUERY
    const tours = await query
    res.status(200).json({
      status: 'success',
      results: tours.length,
      data: {
        tours
      }
    });
  } catch (error) {
    res.status(400).json({
      status: 'fail',
      message: error
    })
  }
};

Test Case

http://localhost:3000/api/v1/tours?fields=name,durations,price, Query the data which only return name, durations and price
http://localhost:3000/api/v1/tours?limit=10&page=2, Query the data which skip the first 10 results

Explanation

Field Limiting (start from line 22)

If the ‘fields’ query paramater is existed, split the parameter from comma to white-space so that it can read and limit multiple fields
Apply mongoose select methods to only return the selected field.
Exclude out the mongo version field ‘__v’ by simply put a negative sign (-) infront

Pagination (start from line 31)

Pagination is very useful implementation when you are dealing with a very large dataset.
Combining .skip and .limit methods allow us to achieve pagination.
- .skip: total records we wanted to skip from the query
- .limit: total records we wanted to show from the query
As we are passing the Page Number page and Total Records limit from the query parameters. We can then come out with the formula
- const skip = (page - 1) * limit. This will let us know how mnay records we are required to skip and paginate to our desire page number