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Aggregation Pipeline Quick Reference

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For details on specific operator, including syntax and examples, click on the specific operator to go to its reference page.

Stages

In the db.collection.aggregate method, pipeline stages appear in an array. Documents pass through the stages in sequence. All except the $out and $geoNear stages can appear multiple times in a pipeline.

db.collection.aggregate( [ { <stage> }, ... ] )
Name Description
$addFields Adds new fields to documents. Outputs documents that contain all existing fields from the input documents and newly added fields.
$bucket Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.
$bucketAuto Categorizes incoming documents into a specific number of groups, called buckets, based on a specified expression. Bucket boundaries are automatically determined in an attempt to evenly distribute the documents into the specified number of buckets.
$collStats Returns statistics regarding a collection or view.
$count Returns a count of the number of documents at this stage of the aggregation pipeline.
$currentOp Returns information on active and/or dormant operations for the MongoDB deployment. To run, use the db.aggregate() method.
$facet Processes multiple aggregation pipelines within a single stage on the same set of input documents. Enables the creation of multi-faceted aggregations capable of characterizing data across multiple dimensions, or facets, in a single stage.
$geoNear Returns an ordered stream of documents based on the proximity to a geospatial point. Incorporates the functionality of $match, $sort, and $limit for geospatial data. The output documents include an additional distance field and can include a location identifier field.
$graphLookup Performs a recursive search on a collection. To each output document, adds a new array field that contains the traversal results of the recursive search for that document.
$group Groups input documents by a specified identifier expression and applies the accumulator expression(s), if specified, to each group. Consumes all input documents and outputs one document per each distinct group. The output documents only contain the identifier field and, if specified, accumulated fields.
$indexStats Returns statistics regarding the use of each index for the collection.
$limit Passes the first n documents unmodified to the pipeline where n is the specified limit. For each input document, outputs either one document (for the first n documents) or zero documents (after the first n documents).
$listLocalSessions Lists all active sessions recently in use on the currently connected mongos or mongod instance. These sessions may have not yet propagated to the system.sessions collection.
$listSessions Lists all sessions that have been active long enough to propagate to the system.sessions collection.
$lookup Performs a left outer join to another collection in the same database to filter in documents from the “joined” collection for processing.
$match Filters the document stream to allow only matching documents to pass unmodified into the next pipeline stage. $match uses standard MongoDB queries. For each input document, outputs either one document (a match) or zero documents (no match).
$out Writes the resulting documents of the aggregation pipeline to a collection. To use the $out stage, it must be the last stage in the pipeline.
$project Reshapes each document in the stream, such as by adding new fields or removing existing fields. For each input document, outputs one document.
$redact Reshapes each document in the stream by restricting the content for each document based on information stored in the documents themselves. Incorporates the functionality of $project and $match. Can be used to implement field level redaction. For each input document, outputs either one or zero documents.
$replaceRoot Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the _id field. Specify a document embedded in the input document to promote the embedded document to the top level.
$sample Randomly selects the specified number of documents from its input.
$skip Skips the first n documents where n is the specified skip number and passes the remaining documents unmodified to the pipeline. For each input document, outputs either zero documents (for the first n documents) or one document (if after the first n documents).
$sort Reorders the document stream by a specified sort key. Only the order changes; the documents remain unmodified. For each input document, outputs one document.
$sortByCount Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.
$unwind Deconstructs an array field from the input documents to output a document for each element. Each output document replaces the array with an element value. For each input document, outputs n documents where n is the number of array elements and can be zero for an empty array.

Expressions

Expressions can include field paths and system variables, literals, expression objects, and expression operators. Expressions can be nested.

Field Path and System Variables

Aggregation expressions use field path to access fields in the input documents. To specify a field path, use a string that prefixes with a dollar sign $ the field name or the dotted field name, if the field is in embedded document. For example, "$user" to specify the field path for the user field or "$user.name" to specify the field path to "user.name" field.

"$<field>" is equivalent to "$$CURRENT.<field>" where the CURRENT is a system variable that defaults to the root of the current object in the most stages, unless stated otherwise in specific stages. CURRENT can be rebound.

Along with the CURRENT system variable, other system variables are also available for use in expressions. To use user-defined variables, use $let and $map expressions. To access variables in expressions, use a string that prefixes the variable name with $$.

Literals

Literals can be of any type. However, MongoDB parses string literals that start with a dollar sign $ as a path to a field and numeric/boolean literals in expression objects as projection flags. To avoid parsing literals, use the $literal expression.

Expression Objects

Expression objects have the following form:

{ <field1>: <expression1>, ... }

If the expressions are numeric or boolean literals, MongoDB treats the literals as projection flags (e.g. 1 or true to include the field), valid only in the $project stage. To avoid treating numeric or boolean literals as projection flags, use the $literal expression to wrap the numeric or boolean literals.

Accumulators

Available for use in the $group stage, accumulators are operators that maintain their state (e.g. totals, maximums, minimums, and related data) as documents progress through the pipeline.

When used as accumulators in the $group stage, these operators take as input a single expression, evaluating the expression once for each input document, and maintain their stage for the group of documents that share the same group key.

Changed in version 3.2: Some operators that are available as accumulators for the $group stage are also available for use in the $project stage but not as accumulators. When used in the $project stage, these operators do not maintain their state and can take as input either a single argument or multiple arguments.

Operator Expressions

Operator expressions are similar to functions that take arguments. In general, these expressions take an array of arguments and have the following form:

{ <operator>: [ <argument1>, <argument2> ... ] }

If operator accepts a single argument, you can omit the outer array designating the argument list:

{ <operator>: <argument> }

To avoid parsing ambiguity if the argument is a literal array, you must wrap the literal array in a $literal expression or keep the outer array that designates the argument list.

Name Description
$abs Returns the absolute value of a number.
$add Adds numbers to return the sum, or adds numbers and a date to return a new date. If adding numbers and a date, treats the numbers as milliseconds. Accepts any number of argument expressions, but at most, one expression can resolve to a date.
$addToSet

Returns an array of unique expression values for each group. Order of the array elements is undefined.

Available in $group stage only.

$allElementsTrue Returns true if no element of a set evaluates to false, otherwise, returns false. Accepts a single argument expression.
$and Returns true only when all its expressions evaluate to true. Accepts any number of argument expressions.
$anyElementTrue Returns true if any elements of a set evaluate to true; otherwise, returns false. Accepts a single argument expression.
$arrayElemAt Returns the element at the specified array index.
$arrayToObject Converts an array of key value pairs to a document.
$avg

Returns an average of numerical values. Ignores non-numeric values.

Changed in version 3.2: Available in both $group and $project stages.

$ceil Returns the smallest integer greater than or equal to the specified number.
$cmp Returns: 0 if the two values are equivalent, 1 if the first value is greater than the second, and -1 if the first value is less than the second.
$concat Concatenates any number of strings.
$concatArrays Concatenates arrays to return the concatenated array.
$cond A ternary operator that evaluates one expression, and depending on the result, returns the value of one of the other two expressions. Accepts either three expressions in an ordered list or three named parameters.
$dateFromParts Constructs a BSON Date object given the date’s constituent parts.
$dateToParts Returns a document containing the constituent parts of a date.
$dateFromString Returns a date/time as a date object.
$dateToString Returns the date as a formatted string.
$dayOfMonth Returns the day of the month for a date as a number between 1 and 31.
$dayOfWeek Returns the day of the week for a date as a number between 1 (Sunday) and 7 (Saturday).
$dayOfYear Returns the day of the year for a date as a number between 1 and 366 (leap year).
$divide Returns the result of dividing the first number by the second. Accepts two argument expressions.
$eq Returns true if the values are equivalent.
$exp Raises e to the specified exponent.
$filter Selects a subset of the array to return an array with only the elements that match the filter condition.
$first

Returns a value from the first document for each group. Order is only defined if the documents are in a defined order.

Available in $group stage only.

$floor Returns the largest integer less than or equal to the specified number.
$gt Returns true if the first value is greater than the second.
$gte Returns true if the first value is greater than or equal to the second.
$hour Returns the hour for a date as a number between 0 and 23.
$ifNull Returns either the non-null result of the first expression or the result of the second expression if the first expression results in a null result. Null result encompasses instances of undefined values or missing fields. Accepts two expressions as arguments. The result of the second expression can be null.
$in Returns a boolean indicating whether a specified value is in an array.
$indexOfArray Searches an array for an occurence of a specified value and returns the array index of the first occurence. If the substring is not found, returns -1.
$indexOfBytes Searches a string for an occurence of a substring and returns the UTF-8 byte index of the first occurence. If the substring is not found, returns -1.
$indexOfCP Searches a string for an occurence of a substring and returns the UTF-8 code point index of the first occurence. If the substring is not found, returns -1.
$isArray Determines if the operand is an array. Returns a boolean.
$isoDayOfWeek Returns the weekday number in ISO 8601 format, ranging from 1 (for Monday) to 7 (for Sunday).
$isoWeek Returns the week number in ISO 8601 format, ranging from 1 to 53. Week numbers start at 1 with the week (Monday through Sunday) that contains the year’s first Thursday.
$isoWeekYear Returns the year number in ISO 8601 format. The year starts with the Monday of week 1 (ISO 8601) and ends with the Sunday of the last week (ISO 8601).
$last

Returns a value from the last document for each group. Order is only defined if the documents are in a defined order.

Available in $group stage only.

$let

Defines variables for use within the scope of a subexpression and returns the result of the subexpression. Accepts named parameters.

Accepts any number of argument expressions.

$literal Return a value without parsing. Use for values that the aggregation pipeline may interpret as an expression. For example, use a $literal expression to a string that starts with a $ to avoid parsing as a field path.
$ln Calculates the natural log of a number.
$log Calculates the log of a number in the specified base.
$log10 Calculates the log base 10 of a number.
$lt Returns true if the first value is less than the second.
$lte Returns true if the first value is less than or equal to the second.
$map Applies a subexpression to each element of an array and returns the array of resulting values in order. Accepts named parameters.
$max

Returns the highest expression value for each group.

Changed in version 3.2: Available in both $group and $project stages.

$mergeObjects Combines multiple documents into a single document.
$meta Access text search metadata.
$min

Returns the lowest expression value for each group.

Changed in version 3.2: Available in both $group and $project stages.

$millisecond Returns the milliseconds of a date as a number between 0 and 999.
$minute Returns the minute for a date as a number between 0 and 59.
$mod Returns the remainder of the first number divided by the second. Accepts two argument expressions.
$month Returns the month for a date as a number between 1 (January) and 12 (December).
$multiply Multiplies numbers to return the product. Accepts any number of argument expressions.
$ne Returns true if the values are not equivalent.
$not Returns the boolean value that is the opposite of its argument expression. Accepts a single argument expression.
$objectToArray Converts a document to an array of documents representing key-value pairs.
$or Returns true when any of its expressions evaluates to true. Accepts any number of argument expressions.
$pow Raises a number to the specified exponent.
$push

Returns an array of expression values for each group.

Available in $group stage only.

$range Outputs an array containing a sequence of integers according to user-defined inputs.
$reduce Applies an expression to each element in an array and combines them into a single value.
$reverseArray Returns an array with the elements in reverse order.
$second Returns the seconds for a date as a number between 0 and 60 (leap seconds).
$setDifference Returns a set with elements that appear in the first set but not in the second set; i.e. performs a relative complement of the second set relative to the first. Accepts exactly two argument expressions.
$setEquals Returns true if the input sets have the same distinct elements. Accepts two or more argument expressions.
$setIntersection Returns a set with elements that appear in all of the input sets. Accepts any number of argument expressions.
$setIsSubset Returns true if all elements of the first set appear in the second set, including when the first set equals the second set; i.e. not a strict subset. Accepts exactly two argument expressions.
$setUnion Returns a set with elements that appear in any of the input sets.
$size Returns the number of elements in the array. Accepts a single expression as argument.
$slice Returns a subset of an array.
$split Splits a string into substrings based on a delimiter. Returns an array of substrings. If the delimiter is not found within the string, returns an array containing the original string.
$sqrt Calculates the square root.
$stdDevPop

Returns the population standard deviation of the input values.

Changed in version 3.2: Available in both $group and $project stages.

$stdDevSamp

Returns the sample standard deviation of the input values.

Changed in version 3.2: Available in both $group and $project stages.

$strcasecmp Performs case-insensitive string comparison and returns: 0 if two strings are equivalent, 1 if the first string is greater than the second, and -1 if the first string is less than the second.
$strLenBytes Returns the number of UTF-8 encoded bytes in a string.
$strLenCP Returns the number of UTF-8 code points in a string.
$substr Deprecated. Use $substrBytes or $substrCP.
$substrBytes Returns the substring of a string. Starts with the character at the specified UTF-8 byte index (zero-based) in the string and continues for the specified number of bytes.
$substrCP Returns the substring of a string. Starts with the character at the specified UTF-8 code point (CP) index (zero-based) in the string and continues for the number of code points specified.
$subtract Returns the result of subtracting the second value from the first. If the two values are numbers, return the difference. If the two values are dates, return the difference in milliseconds. If the two values are a date and a number in milliseconds, return the resulting date. Accepts two argument expressions. If the two values are a date and a number, specify the date argument first as it is not meaningful to subtract a date from a number.
$sum

Returns a sum of numerical values. Ignores non-numeric values.

Changed in version 3.2: Available in both $group and $project stages.

$switch Evaluates a series of case expressions. When it finds an expression which evaluates to true, $switch executes a specified expression and breaks out of the control flow.
$toLower Converts a string to lowercase. Accepts a single argument expression.
$toUpper Converts a string to uppercase. Accepts a single argument expression.
$trunc Truncates a number to its integer.
$type Return the BSON data type of the field.
$week Returns the week number for a date as a number between 0 (the partial week that precedes the first Sunday of the year) and 53 (leap year).
$year Returns the year for a date as a number (e.g. 2014).
$zip Merge two arrays together.