1. Join stage
$lookup performs a left outer join, matching localField to foreignField in another collection.
{ $lookup: { from: "users", localField: "userId", foreignField: "_id", as: "user" } }Typorithm · medium
$lookup & $unwind
mongo-lookup-unwindmongodbjoinunwindarrays
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Step 1 of 8 · intro card
Learn $lookup & $unwind
Aggregation
$lookup joins another collection, putting matches into an array field, and $unwind flattens that array into one document per element so the joined data is easy to use.
$lookup is MongoDB's left outer join: it matches localField against foreignField in the from collection and stores results in an array named by as.
Because matches land in an array, even a one-to-one join leaves you with a single-element array under that field.
$unwind flattens the array, producing one output document per array element, so a downstream stage can read joined fields directly.
01
$lookup is a left outer join into an array field.
02
localField matches foreignField across collections.
03
$unwind flattens the array to one doc per element.
db.orders.aggregate([{ $lookup: { from: "users", localField: "userId", foreignField: "_id", as: "user" } }, { $unwind: "$user" }])Read the implementation
Connect each part of the code to the concept before you type.
2. Target collection
from names the foreign collection to join against.
from: "users"3. Join keys
localField on the input is matched to foreignField on the foreign collection.
localField: "userId", foreignField: "_id"4. Output array
as names the new array field that receives the matched documents.
as: "user"5. Flatten stage
$unwind turns the array field into one document per element, flattening the single-match join.
{ $unwind: "$user" }A complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.