What is the $lookup stage in aggregation?

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Accepted Answer

$lookup is the aggregation stage that performs a left outer join with another collection. The closest thing MongoDB has to a SQL JOIN. Inputs — from (other collection), localField , foreignField , as (output array name). Output — matched documents are added as an array under the as field. Sub-pipeline form — let + pipeline for complex join conditions and projections. Performance caveat — can be slow on sharded collections; index the foreign field. Use sparingly — if you constantly need $lookup , the data model probably belongs embedded. $lookup = LEFT OUTER JOIN. Always produces an array (use $unwind for one-to-one). Basic syntax for simple field equality. Pipeline syntax for complex conditions (multiple fields, filters). Performance: $lookup is expensive on large collections — consider embedding for frequently joined data.