$unwind for flattening arrays in MongoDB - Step-by-Step Execution

Concept Flow - $unwind for flattening arrays

Start with document

↓

Check array field

↓

For each element in array

↓

Create new document with element

↓

Output each new document

↓

Repeat for all documents

The $unwind stage takes each document with an array field and outputs a separate document for each element in that array, flattening the array.

Execution Sample

MongoDB

db.orders.aggregate([
  { $unwind: "$items" }
])

This pipeline takes each order document and outputs one document per item in the items array.

Execution Table

Step	Input Document	Array Field	Unwind Action	Output Documents
1	{"_id":1, "items":["apple", "banana", "cherry"]}	["apple", "banana", "cherry"]	Take each element in items array	{"_id":1, "items":"apple"}, {"_id":1, "items":"banana"}, {"_id":1, "items":"cherry"}
2	{"_id":2, "items":["orange"]}	["orange"]	Take each element in items array	{"_id":2, "items":"orange"}
3	{"_id":3, "items":[]}	[]	No elements to unwind, document removed	No output document
4	{"_id":4, "items":null}	null	Field is null, document removed	No output document
5	No more documents	-	End of input	-

💡 All input documents processed; documents with empty or null arrays produce no output.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Current Document	None	{"_id":1, "items":["apple", "banana", "cherry"]}	{"_id":2, "items":["orange"]}	{"_id":3, "items":[]}	{"_id":4, "items":null}	None
Array Elements	None	["apple", "banana", "cherry"]	["orange"]	[]	null	None
Output Documents	[]	[{"_id":1, "items":"apple"}, {"_id":1, "items":"banana"}, {"_id":1, "items":"cherry"}]	[{"_id":2, "items":"orange"}]	[]	[]	[]

Key Moments - 3 Insights

Why does a document with an empty array produce no output?

Because $unwind outputs one document per array element, if the array is empty, there are no elements to output, so the document is removed (see execution_table row 3).

What happens if the array field is null or missing?

If the field is null or missing, $unwind removes the document by default (see execution_table row 4).

How does $unwind handle multiple elements in the array?

It creates one output document per element, each with the same original fields but the array field replaced by the single element (see execution_table row 1).

Visual Quiz - 3 Questions

Look at the execution table, what is the output for the document with _id 2?

A {"_id":2, "items":["orange"]}

B No output document

C {"_id":2, "items":"orange"}

D Document removed

💡 Hint

Check execution_table row 2 under Output Documents

At which step does the condition of an empty array cause no output?

A Step 3

B Step 1

C Step 4

D Step 2

💡 Hint

Look at execution_table row 3 where Array Field is []

If the array field was missing instead of null in document _id 4, what would happen?

A Document would be output with array field missing

B Document would be removed, no output

C Document would output one document with null

D Error occurs

💡 Hint

Refer to key_moments about null or missing fields behavior

Concept Snapshot

$unwind stage syntax:
{ $unwind: "<arrayField>" }

Behavior:
- Outputs one document per element in the array field
- Removes documents with empty or null arrays by default
- Flattens arrays for easier processing in aggregation

Full Transcript

The $unwind stage in MongoDB aggregation takes each document with an array field and creates separate documents for each element in that array. For example, if a document has an items array with three elements, $unwind outputs three documents, each with one item. Documents with empty arrays or null array fields are removed by default. This helps flatten nested arrays for easier querying and processing. The execution table shows step-by-step how each input document is processed and what output documents are produced or removed.