What does $unwind do, and what silent behaviour should you know about?

Short answer

It turns each array element into its own document, copying the parent fields. That is how you aggregate across line items. By default, documents with a missing or empty array disappear from the stream.

Why?

preserveNullAndEmptyArrays keeps those parents. Because $unwind multiplies document count, $match before $unwind is usually the difference between a fast and a slow pipeline. After unwind you $group on the element fields.

Example

db.orders.aggregate([
  { $unwind: "$items" },
  {
    $group: {
      _id: "$items.name",
      unitsSold: { $sum: "$items.quantity" }
    }
  }
])

Interview tip

Mention empty-array documents vanishing. It is a favourite follow-up.

Common mistake

Unwinding first on a 50 million order collection, then matching status.