MongoDB 按数组内的字符串字段分组

Question

I did a lot of research on MongoDB aggregation grouping, but couldn't find a solution.

I have the following document structure:

{
  "_id":"6053e0ef22b8e60015a23da8",
  "shoppinglist":[
     {
        "_id":"606ae2e34e4416001538104f",
        "items":[
           {
              "_id":"6071c5ed8f669f0015e6eebe",
              "product_id":"605852c28ea29f0015653d6f",
           },
           ...
        ]
     }
}

My goal is to group the items in each shopping list object using the product_id , so that my result looks like this:

{
  "_id":"6053e0ef22b8e60015a23da8",
  "shoppinglist":[
     {
        "_id":"606ae2e34e4416001538104f",
        "items":[
           {
              "_id":"6071c5ed8f669f0015e6eebe",
              "product_id":"605852c28ea29f0015653d6f",
              "count": 3 //3 items with the product_id = 605852c28ea29f0015653d6f
           },
           ...
        ]
     }
}

Can someone help me with this, I'm desperate.

Answer 1

• $unwind deconstruct shoppinglist array
• $unwind deconstruct shoppinglist.items array
• $group by _id and product_id , and get required fields using $first and get count using $sum
• $group by _id and shoppinglist._id and reconstruct array of items
• $group by _id and reconstruct array of shoppinglist

db.collection.aggregate([
  { $unwind: "$shoppinglist" },
  { $unwind: "$shoppinglist.items" },
  {
    $group: {
      _id: {
        _id: "$_id",
        product_id: "$shoppinglist.items.product_id"
      },
      shoppinglist_id: { $first: "$shoppinglist._id" },
      items_id: { $first: "$shoppinglist.items._id" },
      count: { $sum: 1 }
    }
  },
  {
    $group: {
      _id: {
        _id: "$_id._id",
        shoppinglist_id: "$shoppinglist_id"
      },
      items: {
        $push: {
          items_id: "$items_id",
          product_id: "$_id.product_id",
          count: "$count"
        }
      }
    }
  },
  {
    $group: {
      _id: "$_id._id",
      shoppinglist: {
        $push: {
          _id: "$_id.shoppinglist_id",
          items: "$items"
        }
      }
    }
  }
])

Playground