How to check if there is a key in collection that has more than one value?

Question

My collection looks something like this:

{
  {
    _id: 'some value',
    'product/productId': 'some value',
    'product/title': 'some value',
    'product/price': 'unknown'
  },
  {
    _id: 'some value',
    'product/productId': 'some value',
    'product/title': 'some value',
    'product/price': '12.57'
  }
}

My goal is to find if there are any products that have more than one price. Values of the key "product/price" can be "unknown" or numerical (e.g. "12.75"). Is there a way to write an aggregation pipeline for that or do I need to use a map-reduce algorithm? I tried both options but didn't find the solution.

Answer 1

If I've understood correctly you can try this aggregation pipeline:

First of all, the _id field is (or should be) unique, so I think you mean another field like id.

So the trick is to group by that id and get all prices into an array. Then filter using $match to get only documents where the total of prices is greater than 1.

db.collection.aggregate([
  {
    "$group": {
      "_id": "$id",
      "price": {
        "$push": "$product/price"
      }
    }
  },
  {
    "$match": {
      "$expr": {
        "$gt": [ { "$size": "$price" }, 1 ]
      }
    }
  }
])

Example here

As added into comments for Joe if you want consider identical values as the same you have to use $addToSet

Example here