MongoDB 合并来自不同集合的两个聚合

Question

I am new to backend and MongoDB so forgive me if my question sounds stupid.

I have two collections of offers and products detailed below

Products

[
  {
    "name": "product #1",
    "place": "place ID"
  },
  {
    "name": "product #2",
    "place": "place ID"
  },
  {
    "name": "product #3",
    "place": "place ID"
  }
]

Offers

[
  {
    "name": "offer #1",
    "place": "place ID"
  },
  {
    "name": "offer #2",
    "place": "place ID"
  },
  {
    "name": "offer #3",
    "place": "place ID"
  }
]

so my target is to merge the two aggregations of these two collections and get output similar to this one

[
  {
    "name": "offer #1",
    "place": "place ID",
    "type": "offer"
  },
  {
    "name": "product #1",
    "place": "place ID",
    "type": "product"
  },
  {
    "name": "offer #2",
    "place": "place ID",
    "type": "offer"
  },
  {
    "name": "offer #3",
    "place": "place ID",
    "type": "offer"
  },
  {
    "name": "product #2",
    "place": "place ID",
    "type": "product"
  },
  {
    "name": "product #3",
    "place": "place ID",
    "type": "product"
  }
]

should I aggregate on the product and offer collections or aggregate on place and group the result.

Answer 1

You can try,

• $facet to add all products in a array and add type field
• $lookup with offers
• $project to add type field in offers $map through and concat both product and offer using $concatArrays
• $unwind deconstruct final array
• $replaceRoot to replace final object in root

db.product.aggregate([
  {
    $facet: {
      product: [
        { $addFields: { type: "product" } }
      ]
    }
  },
  {
    $lookup: {
      from: "offer",
      as: "offer",
      pipeline: []
    }
  },
  {
    $project: {
      final: {
        $concatArrays: [
          "$product",
          {
            $map: {
              input: "$offer",
              in: { $mergeObjects: ["$$this", { type: "offer" }] }
            }
          }
        ]
      }
    }
  },
  { $unwind: "$final" },
  { $replaceRoot: { newRoot: "$final" } }
])

Playground