Querying arrays

Arrays behave in a way that surprises people coming from SQL: matching an array field against a single value asks “does any element equal this?”. That one rule explains most array queries you will write.

Suppose:

{
  name: "Denish",
  skills: ["Node.js", "MongoDB", "Kafka"]
}

Find people with MongoDB:

db.users.find({
  skills: "MongoDB"
})

MongoDB automatically matches an element inside the array.

You don’t need:

{
  skills: {
    $contains: "MongoDB"
  }
}

That doesn’t exist.

Find documents containing any of these

db.users.find({
  skills: {
    $in: ["MongoDB", "Redis"]
  }
})

Meaning:

User has MongoDB OR Redis.

Find documents containing all of these

db.users.find({
  skills: {
    $all: ["MongoDB", "Redis"]
  }
})

Meaning:

User must have both.