$project stage for shaping output in MongoDB - Step-by-Step Execution

Concept Flow - $project stage for shaping output

Input Documents

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$project Stage

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Select Fields

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Rename or Create Fields

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Output Documents with New Shape

The $project stage takes input documents, selects or reshapes fields, and outputs documents with the new structure.

Execution Sample

MongoDB

db.orders.aggregate([
  { $project: { item: 1, total: { $multiply: ["$price", "$quantity"] } } }
])

This pipeline reshapes each order document to keep 'item' and add a new 'total' field calculated by multiplying price and quantity.

Execution Table

Step	Input Document	Action	Projected Fields	Output Document
1	{"item": "apple", "price": 2, "quantity": 5, "category": "fruit"}	Keep 'item', compute 'total' = price * quantity	{"item": "apple", "total": 10}	{"item": "apple", "total": 10}
2	{"item": "banana", "price": 1, "quantity": 10, "category": "fruit"}	Keep 'item', compute 'total' = price * quantity	{"item": "banana", "total": 10}	{"item": "banana", "total": 10}
3	{"item": "carrot", "price": 3, "quantity": 4, "category": "vegetable"}	Keep 'item', compute 'total' = price * quantity	{"item": "carrot", "total": 12}	{"item": "carrot", "total": 12}
4	No more documents	End of pipeline	N/A	Pipeline ends

💡 All input documents processed, pipeline ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Input Document	N/A	{"item": "apple", "price": 2, "quantity": 5, "category": "fruit"}	{"item": "banana", "price": 1, "quantity": 10, "category": "fruit"}	{"item": "carrot", "price": 3, "quantity": 4, "category": "vegetable"}	No more documents
Projected Fields	N/A	{"item": "apple", "total": 10}	{"item": "banana", "total": 10}	{"item": "carrot", "total": 12}	N/A
Output Document	N/A	{"item": "apple", "total": 10}	{"item": "banana", "total": 10}	{"item": "carrot", "total": 12}	Pipeline ends

Key Moments - 2 Insights

Why does the output document only have 'item' and 'total' fields, not 'price' or 'quantity'?

Because the $project stage explicitly includes only 'item' and the computed 'total' field, all other fields are excluded by default as shown in execution_table rows 1-3.

How is the 'total' field calculated in each output document?

The 'total' field is computed by multiplying the 'price' and 'quantity' fields from the input document, as seen in the Action column of execution_table rows 1-3.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. What is the value of the 'total' field in the output document?

A 10

B 1

C 11

D 0

💡 Hint

Check the 'Projected Fields' column at Step 2 in the execution_table.

At which step does the pipeline finish processing all documents?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look for the row where 'No more documents' is in the Input Document column.

If we added 'category: 1' to the $project stage, what would change in the output documents?

A The output documents would include the 'category' field as well.

B The 'total' field would be removed.

C The input documents would change.

D Nothing would change.

💡 Hint

Refer to how $project controls which fields appear in output documents in the execution_table.

Concept Snapshot

$project stage reshapes documents in aggregation.
Specify fields to keep with 1, exclude others.
Create new fields with expressions.
Output documents have only projected fields.
Useful for selecting, renaming, or computing fields.

Full Transcript

The $project stage in MongoDB aggregation takes each input document and reshapes it by selecting specific fields to keep or exclude. It can also create new fields using expressions like multiplication. In the example, the pipeline keeps the 'item' field and adds a new 'total' field calculated by multiplying 'price' and 'quantity'. Each input document is processed step-by-step, producing output documents with only the projected fields. The pipeline ends after all documents are processed.