block4.js

var doc = db.
  getCollection('customers').
  aggregate(
    [
      {
        $match: { _id: customerId }
      },
      {
        $lookup: {
          from: 'orders',
          localField: '_id',
          foreignField: 'customerId',
          as: 'orders'
        }
      },
      {
        $lookup: {
          from: 'lineitems',
          localField: 'orders._id',
          foreignField: 'orderId',
          as: 'lineitems'
        }
      },
      {
        $lookup: {
          from: 'products',
          localField: 'lineitems.prodId',
          foreignField: '_id',
          as: 'products'
        }
      }
    ],
    { allowDiskUse: true }
  )