How to get elements of an array which is not in another array in MongoDB Aggregation Pipeline?

Question

I have two arrays as below:

LastOneYearCustomerIds:[1,2,3,4,5,6,7,8]
ThisMonthCustomerIds:[1,2,3,4,9,10]

I need to find the New Customer Ids which were not in previous year. I tried creating one pipeline in MongoDB Compass but it will give the difference, I am looking for something which can return me elements in ThisMonthCustomerIds but not in LastOneYearCustomerIds. I also tried following other posts on stack overflow but couldn't find a relevant solution.

Expected Result is:

NewCustomerIds:[9,10]

I have tried below Aggregation pipeline which will give me the difference but not new CustomerIds:

$project: {
  newCustomerIds:{
    $setDifference:
    ['$LastOneYearCustomerIds','$ThisMonthCustomerIds'
  ]}
}

hhharsha36 · Accepted Answer · 2020-07-13 06:30:51Z

2

The $setDifference operator will work. You have placed the key names in the wrong order.

The array whose elements you want to preserve should come first, followed by the array you want to compare to.

db.test10.aggregate([
    {
        "$project": {
            "newCustomerIds": {
                "$setDifference": ["$ThisMonthCustomerIds", "$LastOneYearCustomerIds"]
            }
        }
    }
])

The above query will return the output:

{
    "_id" : ObjectId("5f0bfcc55e654d34080a9282"),
    "newCustomerIds" : [
        9,
        10
    ]
}

answered Jul 13, 2020 at 6:30

hhharsha36

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1 Comment

Bersan Over a year ago

Is there a way to do this using $filter, something like $filter: { input: "LastOneYearCustomerIds", cond: $not { $in: ["$$this", "ThisMonthCustomerIds"]}}? I'm trying this alternative but getting a weird syntax error

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