MongoDB - How to tell if every document has an exactly matching element array after an aggregate match

Question

I have a collection of documents similar to this:

{
    Name : "Name1",
    Product : 1012,
    Titles : [ { 
         Id: 5,  
         Title: "FirstTitle" 
         }, 
         {
         Id: 75,
         Title: "SecondTitle"
         }
 },
 {
    Name : "Name1",
    Product : 2014,
    Titles : [ { 
         Id: 5,  
         Title: "FirstTitle" 
         }, 
         {
         Id: 75,
         Title: "SecondTitle"
         }
 }

I'm matching by the Name with an aggregate to get all documents with the same name. Then if all matches have the exact same set of Titles, I want that set.

{
    Name : "Name1,
    TitlesVaries : false
    Titles : [ { 
         Id: 5,  
         Title: "FirstTitle" 
         }, 
         {
         Id: 75,
         Title: "SecondTitle"
         }
}

If they are different I want to know that.

{
    Name : "Name1"
    TitlesVaries : true
    Titles : null
}

I'm having trouble comparing the set/array titles for each document to see if they are all exactly the same after I've don't my aggregate/match. Some documents can have empty/null arrays of Titles and if they are all empty/null that's a match

Answer 1

The aggreagtion might be

• $addToSet helps to remove duplicates. So we will have two array which is a Set (Titles) and an original array ('original')
• Compare both array. If both are not equal, then there is a various.

The script is

db.collection.aggregate([
  {
    $group: {
      _id: "$Name",
      Titles: { $addToSet: "$Titles" },
      original: { $push: "$Titles" }
    }
  },
  {
    $project: {
      Titles: {
        $cond: [
          {
            $or: [
              {
                $ne: [
                  { $size: "$Titles" },
                  { $size: "$original" }
                ]
              },
              {
                $eq: [ {  $size: "$original"  }, 1 ]
              }
            ]
          },
          "$Titles",
          null
        ]
      },
      TitlesVaries: {
        $ne: [
          { $size: "$Titles" }, { $size: "$original" }
        ]
      }
    }
  }
])

Working Mongo playground