Explore every conceivable pairing of two fields in a Mongo database

I currently have an array of objects stored in my database

[
  {
     price: "1"
     type: "buy",
  },
  {
     price: "2"
     type: "buy",
  },
  {
     price: "3"
     type: "sell"
  },
  {
     price: "4"
     type: "sell"
  }
]

What is the best way to aggregate this data in order to obtain an array of arrays containing all potential combinations (keeping in mind that the price can be any random number)

Filter out items with the highest buy price

and further organize them, so that items with the greatest price difference are positioned at the top

[
  [
      {
         price: "1"
         type: "buy",
      },
      {
         price: "4"
         type: "sell"
      }
  ],
  [
      {
         price: "1"
         type: "buy",
      },
      {
         price: "3"
         type: "sell"
      },
  ],
  [
      {
         price: "2"
         type: "buy",
      },
      {
         price: "4"
         type: "sell"
      }
  ],
  [
      {
         price: "2"
         type: "buy",
      },
      {
         price: "3"
         type: "sell"
      },
  ],
]
javascriptarraysmongodbmongoosemongodb-query

Answer №1

Instructions

  • Perform a self-lookup and only match if the IDs are not the same (to avoid pairing the same item with itself)
  • Use mapping to include the parent item in the final result of the join operation (the pair will always consist of the item with the highest price as the first member, which is necessary for later steps)
  • Unwind the pairs to separate them into individual entities
  • Group the pairs together to remove any duplicates (each pair should only appear twice but they are sorted so we can group by them effectively)
  • Determine the price difference between the paired items, sort them in descending order, and then remove this field from the final result

*It's unclear whether all requirements are met, especially regarding the part about "hiding items with the highest buy price" where the highest buy price is "2" yet those items are still included in the results.

Playmongo

coll.aggregate(
[{"$lookup": 
   {"from": "coll",
    "pipeline": 
     [{"$match": {"$expr": {"$ne": ["$$id", "$_id"]}}},
       {"$project": {"_id": 0, "price": 1, "type": 1}}],
    "as": "pairs",
    "let": {"id": "$_id"}}},
 {"$project": 
   {"_id": 0,
    "pairs": 
     {"$map": 
       {"input": "$pairs",
        "in": 
         {"$cond": 
           [{"$gt": ["$$this.price", "$price"]},
             [{"price": "$price", "type": "$type"}, "$$this"],
             ["$$this", {"price": "$price", "type": "$type"}]]}}}}},
 {"$unwind": "$pairs"}, {"$group": {"_id": "$pairs"}},
 {"$project": {"_id": 0, "pair": "$_id"}},
{"$set"": 
   {"priceDifference": 
     {"$abs": 
       {"$subtract": 
         [{"$toDouble": 
             {"$getField": 
               {"field": "price", "input": {"$arrayElemAt": ["$pairs", 0]}}}},
           {"$toDouble": 
             {"$getField": 
               {"field": "price",
                "input": {"$arrayElemAt": ["$pairs", 1]}}}}]}}}},
 {"$sort": {"priceDifference": -1}},
{"$unset": ["priceDifference"]}])

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