How to fix MongoServerError: PlanExecutor error during aggregation :: caused by :: Exceeded memory limit for $group, but didn't allow external sort in MongoDB

// MongoDB Node.js driver
db.collection('myCollection').aggregate([
  { $group: { _id: '$category', total: { $sum: '$amount' } } }
], { allowDiskUse: true });

// Mongoose
MyModel.aggregate([
  { $group: { _id: '$category', total: { $sum: '$amount' } } }
]).allowDiskUse(true);

// MongoDB shell
db.myCollection.aggregate([
  { $group: { _id: '$category', total: { $sum: '$amount' } } }
], { allowDiskUse: true });

// Instead of accumulating all items:
db.collection.aggregate([
  { $group: { 
      _id: '$userId', 
      items: { $push: '$$ROOT' }  // Stores entire documents
    }
  }
]);

// Only accumulate necessary fields:
db.collection.aggregate([
  { $group: { 
      _id: '$userId', 
      items: { $push: { id: '$_id', name: '$name' } }  // Only needed fields
    }
  }
]);

// Or use $count instead of $push when possible:
db.collection.aggregate([
  { $group: { 
      _id: '$userId', 
      itemCount: { $sum: 1 }
    }
  }
]);

db.collection.aggregate([
  // Filter first to reduce data volume
  { $match: { 
      createdAt: { $gte: new Date('2024-01-01') },
      status: 'active'
    }
  },
  // Then group on smaller dataset
  { $group: { 
      _id: '$category', 
      total: { $sum: '$amount' } 
    }
  }
], { allowDiskUse: true });

# Check your current cluster tier in Atlas dashboard
# Clusters → [Your Cluster] → Configuration

db.collection.aggregate([
  { $bucketAuto: {
      groupBy: '$timestamp',
      buckets: 10,  // Limits to 10 groups
      output: {
        count: { $sum: 1 },
        avgAmount: { $avg: '$amount' }
      }
    }
  }
], { allowDiskUse: true });