Aggregation pipeline
An aggregation pipeline passes documents through a series of stages, each transforming the data.
In this page:
Syntax
db.collection_name.aggregate([
{ $match: filter },
{ $group: { _id: "$field", total: { $sum: 1 } } },
{ $sort: { total: -1 } }
])Aggregation pipeline
db.collection.aggregate([stage1, stage2, ...]) sends documents through stages such as $match, $group, $project and $sort. The output of each stage feeds the next.
Putting $match early reduces the data every later stage must process and lets indexes help.
Note:
Filter with $match as early as possible.
Example: Aggregation pipeline
test> db.sales.insertMany([
... { _id: 1, item: "pen", qty: 5, price: 2 }, { _id: 2, item: "book", qty: 1, price: 12 },
... { _id: 3, item: "pen", qty: 10, price: 2 }, { _id: 4, item: "lamp", qty: 2, price: 30 }
... ])
{
acknowledged: true,
insertedIds: { '0': 1, '1': 2, '2': 3, '3': 4 }
}
test> db.sales.aggregate([
... { $match: { qty: { $gte: 2 } } },
... { $project: { _id: 0, item: 1, total: { $multiply: ["$qty", "$price"] } } },
... { $sort: { total: -1 } }
... ])
[
{ item: 'lamp', total: 60 },
{ item: 'pen', total: 20 },
{ item: 'pen', total: 10 }
]⚠️ Run this in your own terminal or Node.js environment.
Related Topics
Common Mistakes
- Passing stages as separate arguments instead of an array
- Putting $match after an expensive $group
- Forgetting each stage is a separate object
Chapter Summary
- aggregate takes an array of stages
- Each stage feeds the next
- $match early
- Stages are objects with one operator
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