New in version 2.2.
Warning
The pipeline cannot operate on values of the following types: Binary, Symbol, MinKey, MaxKey, DBRef, Code, and CodeWScope.
Pipeline operators appear in an array. Documents pass through the operators in a sequence.
| Name | Description |
|---|---|
| $project | Reshapes a document stream. $project can rename, add, or remove fields as well as create computed values and sub-documents. |
| $match | Filters the document stream, and only allows matching documents to pass into the next pipeline stage. $match uses standard MongoDB queries. |
| $limit | Restricts the number of documents in an aggregation pipeline. |
| $skip | Skips over a specified number of documents from the pipeline and returns the rest. |
| $unwind | Takes an array of documents and returns them as a stream of documents. |
| $group | Groups documents together for the purpose of calculating aggregate values based on a collection of documents. |
| $sort | Takes all input documents and returns them in a stream of sorted documents. |
| $geoNear | Returns an ordered stream of documents based on proximity to a geospatial point. |
Expression operators calculate values within the Pipeline Operators.
| Name | Description |
|---|---|
| $addToSet | Returns an array of all the unique values for the selected field among for each document in that group. |
| $first | Returns the first value in a group. |
| $last | Returns the last value in a group. |
| $max | Returns the highest value in a group. |
| $min | Returns the lowest value in a group. |
| $avg | Returns an average of all the values in a group. |
| $push | Returns an array of all values for the selected field among for each document in that group. |
| $sum | Returns the sum of all the values in a group. |
These operators accept Booleans as arguments and return Booleans as results.
The operators convert non-Booleans to Boolean values according to the BSON standards. Here, null, undefined, and 0 values become false, while non-zero numeric values, and all other types, such as strings, dates, objects become true.
| Name | Description |
|---|---|
| $and | Returns true only when all values in its input array are true. |
| $or | Returns true when any value in its input array are true. |
| $not | Returns the boolean value that is the opposite of the input value. |
These operators perform comparisons between two values and return a Boolean, in most cases reflecting the result of the comparison.
All comparison operators take an array with a pair of values. You may compare numbers, strings, and dates. Except for $cmp, all comparison operators return a Boolean value. $cmp returns an integer.
| Name | Description |
|---|---|
| $cmp | Compares two values and returns the result of the comparison as an integer. |
| $eq | Takes two values and returns true if the values are equivalent. |
| $gt | Takes two values and returns true if the first is larger than the second. |
| $gte | Takes two values and returns true if the first is larger than or equal to the second. |
| $lt | Takes two values and returns true if the second value is larger than the first. |
| $lte | Takes two values and returns true if the second value is larger than or equal to the first. |
| $ne | Takes two values and returns true if the values are not equivalent. |
Arithmetic operators support only numbers.
| Name | Description |
|---|---|
| $add | Computes the sum of an array of numbers. |
| $divide | Takes two numbers and divides the first number by the second. |
| $mod | Takes two numbers and calcualtes the modulo of the first number divided by the second. |
| $multiply | Computes the product of an array of numbers. |
| $subtract | Takes two numbers and subtracts the second number from the first. |
String operators manipulate strings within projection expressions.
| Name | Description |
|---|---|
| $concat | Concatenates two strings. |
| $strcasecmp | Compares two strings and returns an integer that reflects the comparison. |
| $substr | Takes a string and returns portion of that string. |
| $toLower | Converts a string to lowercase. |
| $toUpper | Converts a string to uppercase. |
Date operators take a “Date” typed value as a single argument and return a number.
| Name | Description |
|---|---|
| $dayOfYear | Converts a date to a number between 1 and 366. |
| $dayOfMonth | Converts a date to a number between 1 and 31. |
| $dayOfWeek | Converts a date to a number between 1 and 7. |
| $year | Converts a date to the full year. |
| $month | Converts a date into a number between 1 and 12. |
| $week | Converts a date into a number between 0 and 53 |
| $hour | Converts a date into a number between 0 and 23. |
| $minute | Converts a date into a number between 0 and 59. |
| $second | Converts a date into a number between 0 and 59. May be 60 to account for leap seconds. |
| $millisecond | Returns the millisecond portion of a date as an integer between 0 and 999. |