Population

Population replaces stored ObjectIds with documents from other collections via additional queries. Convenient for references; risky for query count and over-fetching.

					await Order.find()
  .populate({ path: 'customerId', select: 'name email' })
  .lean();
				

On interviews: compare population with embedding, aggregation `$lookup`, manual batched queries, projections, and lean results. Mention cardinality and ownership checks.

Common pitfalls: deep population chains; population making MongoDB feel relational; missing projections and auth checks on referenced docs.

The trade-off is developer ergonomics versus explicit query design.

Checklist:

• Check cardinality before populating.
• Limit projected fields.
• Prefer embedding when aggregate ownership is clear.
• Measure query count per endpoint.