MongoDB group elements in an array

Question

I want, for each product_id, the sum of prices in all orders and another result that I want to achieve is the sum of the price for products in the same array an same id.

\\orders collection:
{
order_id: 123,
customer_id: 1,
order_items: [{product_id:1, price: 2}, {product: 4, price: 8}, {product_id:1, price: 2}]
},
{
order_id: 124,
customer_id: 5,
order_items: [{product_id:5, price: 7}, {product: 4, price: 8}]
}

FIRST result I want:

{product_id: 1, tot_price: 4}, 
{product_id: 4, tot_price: 16},
{product_id: 5, tot_price: 7}

SECOND result:

{order_id:123,
product_tot: [{product_id:1, tot_price: 4}, {product: 4, tot_price: 8}]},
{order_id:124,
product_tot: [{product_id:5, tot_price: 7}, {product: 4, tot_price: 8}},

Answer 1

Try the below queries:

For 1st Result:

db.collection.aggregate([
  {
    $unwind: {
      path: "$order_items",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $group: {
      _id: "$order_items.product_id",
      tot_price: {
        $sum: "$order_items.price"
      }
    }
  },
  {
    $project: {
      _id: 0,
      product_id: "$_id",
      tot_price: 1
    }
  }
])

MongoPlayGroundLink

For 2nd Result:

db.collection.aggregate([
  {
    $unwind: {
      path: "$order_items",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $group: {
      _id: {
        order_id: "$order_id",
        product_id: "$order_items.product_id"
      },
      tot_price: {
        $sum: "$order_items.price"
      }
    }
  },
  {
    $project: {
      _id: 0,
      order_id: "$_id.order_id",
      product_id: "$_id.product_id",
      tot_price: "$tot_price"
    }
  },
  {
    $group: {
      _id: "$order_id",
      product_tot: {
        $push: {
          product_id: "$product_id",
          tot_price: "$tot_price"
        }
      }
    }
  },
  {
    $project: {
      _id: 0,
      order_id: "$_id",
      product_tot: 1
    }
  }
])

MongoPlayGroundLink