$addFields for computed fields in MongoDB - Step-by-Step Execution

Concept Flow - $addFields for computed fields

Start with original documents

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$addFields stage adds new fields

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Compute values using expressions

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Add computed fields to each document

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Output documents with new fields

The $addFields stage takes each document, computes new fields using expressions, and adds these fields to the document.

Execution Sample

MongoDB

db.sales.aggregate([
  { $addFields: { total: { $multiply: ["$price", "$quantity"] } } }
])

This aggregation adds a new field 'total' by multiplying 'price' and 'quantity' in each sales document.

Execution Table

Step	Input Document	Expression Evaluated	Computed Field Value	Output Document
1	{"item": "apple", "price": 2, "quantity": 5}	$multiply: [2, 5]	10	{"item": "apple", "price": 2, "quantity": 5, "total": 10}
2	{"item": "banana", "price": 1, "quantity": 10}	$multiply: [1, 10]	10	{"item": "banana", "price": 1, "quantity": 10, "total": 10}
3	{"item": "orange", "price": 3, "quantity": 7}	$multiply: [3, 7]	21	{"item": "orange", "price": 3, "quantity": 7, "total": 21}
4	No more documents	N/A	N/A	Aggregation ends

💡 All documents processed, aggregation pipeline completes.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
price	varies per doc	2	1	3	N/A
quantity	varies per doc	5	10	7	N/A
total	undefined	10	10	21	N/A

Key Moments - 2 Insights

Why does the 'total' field appear in the output documents?

Because $addFields computes the expression (price * quantity) and adds it as the 'total' field to each document, as shown in execution_table rows 1-3.

What happens if a document is missing 'price' or 'quantity'?

The multiplication expression would result in null or error, so the 'total' field might be missing or null. This is not shown here but important to handle in real cases.

Visual Quiz - 3 Questions

Look at the execution table, what is the 'total' value for the second document?

A 1

B 5

C 10

D 15

💡 Hint

Check execution_table row 2 under 'Computed Field Value'

At which step does the aggregation pipeline finish processing documents?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look at execution_table row 4 where it says 'No more documents'

If the 'quantity' was 0 in a document, what would be the 'total' value?

A null

B 0

C undefined

D 1

💡 Hint

Recall multiplication with zero results in zero, as shown in the expression evaluation column

Concept Snapshot

$addFields adds new fields to documents in aggregation.
Syntax: { $addFields: { newField: <expression> } }
Computes values per document and adds them.
Does not remove existing fields.
Useful for calculated fields like totals or flags.

Full Transcript

The $addFields stage in MongoDB aggregation takes each input document and adds new fields by computing expressions. For example, multiplying price and quantity to get a total. Each document is processed one by one, the expression is evaluated, and the new field is added. This process repeats until all documents are processed. The output documents have all original fields plus the new computed fields. This is useful to enrich data without changing original fields. If a field used in the expression is missing, the result may be null or missing. The execution table shows each step with input document, expression, computed value, and output document. The variable tracker shows how price, quantity, and total change per document. Understanding this helps create dynamic fields in aggregation pipelines.