Embedding vs Referencing

Embed copies the related data into the parent document. Reference stores an id and fetches (or $lookups) the rest. The question is not 'which is MongoDB-like'. It is: is this read together, owned by the parent, and bounded?

// Embed — line items are the order. Bounded. Never shared.
{
  userId: ObjectId("64a1b2c3d4e5f6a7b8c90001"),
  amount: 4900,
  items: [
    { name: "Mechanical Keyboard", quantity: 1, unitPrice: 4500 },
    { name: "USB-C Cable", quantity: 2, unitPrice: 200 }
  ]
}

// Reference — orders are not the user. Unbounded. Listed on their own.
{ name: "Rahul", city: "Bangalore" }
{ userId: ObjectId("64a1b2c3d4e5f6a7b8c90001"), amount: 4900 }

Embed when: 1:1 or 1:few, always fetched with the parent, not reused as a first-class page. Reference when: the child has its own queries (Rahul's order history), many parents would copy a large mutable object, or the array has no natural cap. Hybrid is normal: items embed a snapshot of name and unitPrice, and may still hold productId if you need to join the catalogue later.