Is it possible to create a compound multikey index in MongoDB? The answer is yes, with a very important caveat - only one of the indexed fields can have an array as its value.

For example, suppose we had a document within a students collection with two fields with arrays as their values: sports and clubs.

{ 
  _id: ObjectId(...),
  last_name: "Tapia",
  first_name: "Joseph",
  major: "architecture",
  birth_year: 1988,
  graduation_year: 2012 ,
  sports: ["rowing", "boxing"],
  clubs: [“Honor Society”, “Student Government”, “Yearbook Committee”]
}

A single compound index can not be created on both the sports and clubs fields. We could, however, successfully create a compound multikey index on sports or clubs along with any of the other fields.

For example, either of the following would successfully create a compound multikey index:

db.students.createIndex({ sports: 1, major: 1 });
db.students.createIndex({ clubs: -1, graduation_year: -1 });

If we wanted to index both the sports and clubs fields, we’d have to create two separate indexes for them.

Let’s practice creating a compound multikey index!