MongoDB

Aggregation

Aggregation in MongoDB works like Unix pipeline, one stage will pass to next stage.

$match

$match works just like .find

$project

$project is used to select a subset of the field, 0 means exclude and 1 means include.

We can map value using the syntax:

{$project: {
    "newFieldName": "$firstLevel.second_level"
}}

Metaphorically, $project is like Array.map

$unwind

Split single documents into multiple based on a specific field with array.

Array expression

• use $filter to filter items in an array as part of $project operation.
• use $arrayElemAt to get item at specific index. (negative to get from end)
• use $slice to get subset by index
• use $size operator to get array length

Accumulator

Accumulator examples:

• $sum
• $avg
• $first
• $last
• $max
• $min
• $mergeObjects
• $push
• $addToSet

We can use accumulator in group phase or project phase. When use in project phase, the accumulator can only be applied to array field.