Implement MongoDB $concatArrays even when some of the values are null?

Use the $ifNull operator with $concatArrays in the aggregation framework to concatenate arrays even when some fields are null or missing. The $ifNull operator replaces null values with an empty array, allowing concatenation to proceed successfully.

Syntax

db.collection.aggregate([
  {
    $project: {
      concatenatedField: {
        $concatArrays: [
          { $ifNull: ["$array1", []] },
          { $ifNull: ["$array2", []] }
        ]
      }
    }
  }
]);

Sample Data

db.concatenateArraysDemo.insertMany([
  {
    "FirstSemesterSubjects": ["MongoDB", "MySQL", "Java"],
    "SecondSemesterSubjects": ["C", "C++"]
  },
  {
    "FirstSemesterSubjects": ["C#", "Ruby", "Python"]
  },
  {
    "FirstSemesterSubjects": ["HTML", "CSS", "Javascript"],
    "SecondSemesterSubjects": ["CSS", "Javascript"]
  }
]);

Example: Concatenate Arrays with Null Handling

The following query concatenates arrays even when SecondSemesterSubjects is null or missing ?

db.concatenateArraysDemo.aggregate([
  {
    $project: {
      ConcatenateArrays: {
        $concatArrays: [
          { $ifNull: ["$FirstSemesterSubjects", []] },
          { $ifNull: ["$SecondSemesterSubjects", []] }
        ]
      }
    }
  }
]);

{ "_id" : ObjectId("5cd687707924bb85b3f4895c"), "ConcatenateArrays" : [ "MongoDB", "MySQL", "Java", "C", "C++" ] }
{ "_id" : ObjectId("5cd687927924bb85b3f4895d"), "ConcatenateArrays" : [ "C#", "Ruby", "Python" ] }
{ "_id" : ObjectId("5cd687bb7924bb85b3f4895e"), "ConcatenateArrays" : [ "HTML", "CSS", "Javascript", "CSS", "Javascript" ] }

How It Works

• $ifNull checks if the field is null or missing
• If null/missing, it returns an empty array [] as the fallback
• $concatArrays then concatenates the arrays (including empty ones)
• Documents with missing fields still participate in concatenation

Conclusion

The $ifNull operator ensures that $concatArrays works reliably even with null or missing array fields. This combination prevents aggregation errors and provides consistent array concatenation results across all documents.