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Get indication of values not found in MongoDB

If I use pymongo’s find_one to query for a specific value then if it’s not exist I’ll simply get a None but If I query for multiple values using find then I’ll get a cursor if at least one of the values exists:

pymongo.MongoClient("Conn string")
db = db_client[some_db_name]
collection = db[some_collection_name]
id_name_queries = [(123,'George'),(234,'Jon'),(521,'Tfu')] # queries
clusters = collection.find({"$or": [
                                        {"$and": [{"ID": pair[0]}, {"Name": pair[1]}]} for pair in id_name_queries 
                                        ]
                                })

Is it possible to get from find a boolean list (or other iterable) with the same length of id_name_queries indicating for each query if it exists in the collection?

ind = [True,False,True] # (234,'Jon') is not in the collection

Tags:

2

Answers


  1. 0

    You can use $documents to store all your search entries and start a $lookup to the collection. Check the lookup result for [] to return found or not.

    db.aggregate([
  {
    "$documents": [
      {
        "ID": 123,
        "Name": "George"
      },
      {
        "ID": 234,
        "Name": "Jon"
      },
      {
        "ID": 521,
        "Name": "Tfu"
      }
    ]
  },
  {
    "$lookup": {
      "from": "collection",
      "localField": "ID",
      "foreignField": "ID",
      "let": {
        "name": "$Name"
      },
      "pipeline": [
        {
          "$match": {
            "$expr": {
              "$eq": [
                "$$name",
                "$Name"
              ]
            }
          }
        },
        {
          "$limit": 1
        }
      ],
      "as": "searchResult"
    }
  },
  {
    "$group": {
      "_id": null,
      "result": {
        "$push": {
          $ne: [
            [],
            "$searchResult"
          ]
        }
      }
    }
  }
])

    Mongo Playground for $documents

    • This won’t work. Playground does not support $documents. This is just to demonstrate the syntax.

    Mongo Playground

    • This can demonstrate the actual result of returning boolean array
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  2. 0

    Here’s one way you might do it using aggregate.

    The "$match" stage reduces the collection as much as possible, while the "$group" stage aggregates just the pairs that may be of interest. Finally, "$replaceWith" outputs whether each id_name_queries element is present.

    N.B.: Index(s) on "ID" and/or "Name" may increase query performance. Also, if no "ID" nor any "Name" from id_name_queries is found in the collection, this aggregate will not return a boolean for each id_name_queries element. It’s probably best to test this, observe the output, and decide how you want to handle that possibility.

    clusters = collection.aggregate([
  {
    "$match": {
      "ID": {"$in": [pair[0] for pair in id_name_queries]},
      "Name": {"$in": [pair[1] for pair in id_name_queries]}
    }
  },
  {
    "$group": {
      "_id": null,
      "pairs": {"$addToSet": {"pair": ["$ID","$Name"]}}
    }
  },
  {
    "$replaceWith": {
      "ind": {
        "$reduce": {
          "input": [[pair[0], pair[1]] for pair in id_name_queries],
          "initialValue": [],
          "in": {
            "$concatArrays": [
              "$$value",
              [{"$in": ["$$this","$pairs.pair"]}]
            ]
          }
        }
      }
    }
  }
])

    Try an adapted version of this on mongoplayground.net.

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