expengo_realm_groupAmountByCategories.js

exports = async (input) => {
  // fetching the from and to date from the input query of Custom GraphQL Resolver 
  const {from, to} = input;
  const collection = context.services.get('mongodb-atlas').db('expengo').collection("expenses");
  const user = context.user.id;
  
  // Creating a pipeline that matches the current author and the timeline mentioned in the input query,
  // and then combine all the amount by category. 
  const pipeline = [
    {
      $match: {
        author: new BSON.ObjectId(user), 
        createdAt: {$gte: new Date(from), $lte: new Date(to)}
      }
    }, 
    {
      $group: {
        _id: "$category", 
        amount: {$sum: "$amount"}
      }
    }, 
    {
      $project: {
        category: "$_id", 
        amount: 1, 
        _id: 0
      }
    }
  ];
  const categories = await collection.aggregate(pipeline).toArray();
  return { categories };
};