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Aggregation Pipeline Quick Reference

Note

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

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

db.collection.aggregate( [ { <stage> }, ... ] )
Stage
Description

Adds new fields to documents. Similar to $project, $addFields reshapes each document in the stream; specifically, by adding new fields to output documents that contain both the existing fields from the input documents and the newly added fields.

$set is an alias for $addFields.

Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.
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.
Returns statistics regarding a collection or view.
Returns a count of the number of documents at this stage of the aggregation pipeline.
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.

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.

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.
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.
Returns statistics regarding the use of each index for the collection.
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).
Lists all sessions that have been active long enough to propagate to the system.sessions collection.
Performs a left outer join to another collection in the same database to filter in documents from the "joined" collection for processing.
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).

Writes the resulting documents of the aggregation pipeline to a collection. The stage can incorporate (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) the results into an output collection. To use the $merge stage, it must be the last stage in the pipeline.

New in version 4.2.

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.
Returns plan cache information for a collection.

Reshapes each document in the stream, such as by adding new fields or removing existing fields. For each input document, outputs one document.

See also $unset for removing existing fields.

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.

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.

$replaceWith is an alias for $replaceRoot stage.

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.

$replaceWith is an alias for $replaceRoot stage.

Randomly selects the specified number of documents from its input.

Performs a full-text search of the field or fields in an Atlas collection.

Note

$search is only available for MongoDB Atlas clusters, and is not available for self-managed deployments.

Adds new fields to documents. Similar to $project, $set reshapes each document in the stream; specifically, by adding new fields to output documents that contain both the existing fields from the input documents and the newly added fields.

$set is an alias for $addFields stage.

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).
Reorders the document stream by a specified sort key. Only the order changes; the documents remain unmodified. For each input document, outputs one document.
Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.

Performs a union of two collections; i.e. combines pipeline results from two collections into a single result set.

New in version 4.4.

Removes/excludes fields from documents.

$unset is an alias for $project stage that removes fields.

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.

Starting in version 3.6, MongoDB also provides the db.aggregate() method:

db.aggregate( [ { <stage> }, ... ] )

The following stages use the db.aggregate() method and not the db.collection.aggregate() method.

Stage
Description
Returns information on active and/or dormant operations for the MongoDB deployment.
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.

Starting in MongoDB 4.2, you can use the aggregation pipeline for updates in:

Command
mongo Shell Methods

For the updates, the pipeline can consist of the following stages:

Tip
See also:

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

Aggregation expressions use field path to access fields in the input documents. To specify a field path, prefix the field name or the dotted field name (if the field is in the embedded document) with a dollar sign $. For example, "$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, unless stated otherwise in specific stages.

MongoDB provides various aggregation system variables for use in expressions. To access variables, prefix the variable name with $$. For example:

Variable
Access via $$
Brief Description
$$NOW
Returns the current datetime value, which is same across all members of the deployment and remains constant throughout the aggregation pipeline. (Available in 4.2+)
$$CLUSTER_TIME
Returns the current timestamp value, which is same across all members of the deployment and remains constant throughout the aggregation pipeline. For replica sets and sharded clusters only. (Available in 4.2+)
$$ROOT
References the root document, i.e. the top-level document.
$$CURRENT
References the start of the field path, which by default is ROOT but can be changed.
$$REMOVE
Allows for the conditional exclusion of fields. (Available in 3.6+)
$$DESCEND
One of the allowed results of a $redact expression.
$$PRUNE
One of the allowed results of a $redact expression.
$$KEEP
One of the allowed results of a $redact expression.

For a more detailed description of these variables, see system variables.

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 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.

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.

Arithmetic expressions perform mathematic operations on numbers. Some arithmetic expressions can also support date arithmetic.

Name
Description
Returns the absolute value of a number.
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.
Returns the smallest integer greater than or equal to the specified number.
Returns the result of dividing the first number by the second. Accepts two argument expressions.
Raises e to the specified exponent.
Returns the largest integer less than or equal to the specified number.
Calculates the natural log of a number.
Calculates the log of a number in the specified base.
Calculates the log base 10 of a number.
Returns the remainder of the first number divided by the second. Accepts two argument expressions.
Multiplies numbers to return the product. Accepts any number of argument expressions.
Raises a number to the specified exponent.
Rounds a number to to a whole integer or to a specified decimal place.
Calculates the square root.
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.
Truncates a number to a whole integer or to a specified decimal place.
Name
Description
Returns the element at the specified array index.
Converts an array of key value pairs to a document.
Concatenates arrays to return the concatenated array.
Selects a subset of the array to return an array with only the elements that match the filter condition.
Returns the first array element. Distinct from $first accumulator.
Returns a boolean indicating whether a specified value is in an array.
Searches an array for an occurrence of a specified value and returns the array index of the first occurrence. If the substring is not found, returns -1.
Determines if the operand is an array. Returns a boolean.
Returns the last array element. Distinct from $last accumulator.
Applies a subexpression to each element of an array and returns the array of resulting values in order. Accepts named parameters.
Converts a document to an array of documents representing key-value pairs.
Outputs an array containing a sequence of integers according to user-defined inputs.
Applies an expression to each element in an array and combines them into a single value.
Returns an array with the elements in reverse order.
Returns the number of elements in the array. Accepts a single expression as argument.
Returns a subset of an array.
Merge two arrays together.

Boolean expressions evaluate their argument expressions as booleans and return a boolean as the result.

In addition to the false boolean value, Boolean expression evaluates as false the following: null, 0, and undefined values. The Boolean expression evaluates all other values as true, including non-zero numeric values and arrays.

Name
Description
Returns true only when all its expressions evaluate to true. Accepts any number of argument expressions.
Returns the boolean value that is the opposite of its argument expression. Accepts a single argument expression.
Returns true when any of its expressions evaluates to true. Accepts any number of argument expressions.

Comparison expressions return a boolean except for $cmp which returns a number.

The comparison expressions take two argument expressions and compare both value and type, using the specified BSON comparison order for values of different types.

Name
Description
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.
Returns true if the values are equivalent.
Returns true if the first value is greater than the second.
Returns true if the first value is greater than or equal to the second.
Returns true if the first value is less than the second.
Returns true if the first value is less than or equal to the second.
Returns true if the values are not equivalent.
Name
Description
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.
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.
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.
Name
Description

Defines a custom accumulator function.

New in version 4.4.

Defines a custom function.

New in version 4.4.

The following operators return the size of a data element:

Name
Description
Returns the size of a given string or binary data value's content in bytes.
Returns the size in bytes of a given document (i.e. bsontype Object) when encoded as BSON.

The following operators returns date objects or components of a date object:

Name
Description
Constructs a BSON Date object given the date's constituent parts.
Converts a date/time string to a date object.
Returns a document containing the constituent parts of a date.
Returns the date as a formatted string.
Returns the day of the month for a date as a number between 1 and 31.
Returns the day of the week for a date as a number between 1 (Sunday) and 7 (Saturday).
Returns the day of the year for a date as a number between 1 and 366 (leap year).
Returns the hour for a date as a number between 0 and 23.
Returns the weekday number in ISO 8601 format, ranging from 1 (for Monday) to 7 (for Sunday).
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.
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).
Returns the milliseconds of a date as a number between 0 and 999.
Returns the minute for a date as a number between 0 and 59.
Returns the month for a date as a number between 1 (January) and 12 (December).
Returns the seconds for a date as a number between 0 and 60 (leap seconds).

Converts value to a Date.

New in version 4.0.

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).
Returns the year for a date as a number (e.g. 2014).

The following arithmetic operators can take date operands:

Name
Description
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.
Returns the result of subtracting the second value from the first. 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.
Name
Description
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.
Name
Description

Returns a random float between 0 and 1

New in version 4.4.2.

Randomly select documents at a given rate. Although the exact number of documents selected varies on each run, the quantity chosen approximates the sample rate expressed as a percentage of the total number of documents.

New in version 4.4.2.

Name
Description

Combines multiple documents into a single document.

New in version 3.6.

Converts a document to an array of documents representing key-value pairs.

New in version 3.6.

Set expressions performs set operation on arrays, treating arrays as sets. Set expressions ignores the duplicate entries in each input array and the order of the elements.

If the set operation returns a set, the operation filters out duplicates in the result to output an array that contains only unique entries. The order of the elements in the output array is unspecified.

If a set contains a nested array element, the set expression does not descend into the nested array but evaluates the array at top-level.

Name
Description
Returns true if no element of a set evaluates to false, otherwise, returns false. Accepts a single argument expression.
Returns true if any elements of a set evaluate to true; otherwise, returns false. Accepts a single argument expression.
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.
Returns true if the input sets have the same distinct elements. Accepts two or more argument expressions.
Returns a set with elements that appear in all of the input sets. Accepts any number of argument expressions.
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.
Returns a set with elements that appear in any of the input sets.

String expressions, with the exception of $concat, only have a well-defined behavior for strings of ASCII characters.

$concat behavior is well-defined regardless of the characters used.

Name
Description
Concatenates any number of strings.
Converts a date/time string to a date object.
Returns the date as a formatted string.
Searches a string for an occurrence of a substring and returns the UTF-8 byte index of the first occurrence. If the substring is not found, returns -1.
Searches a string for an occurrence of a substring and returns the UTF-8 code point index of the first occurrence. If the substring is not found, returns -1

Removes whitespace or the specified characters from the beginning of a string.

New in version 4.0.

Applies a regular expression (regex) to a string and returns information on the first matched substring.

New in version 4.2.

Applies a regular expression (regex) to a string and returns information on the all matched substrings.

New in version 4.2.

Applies a regular expression (regex) to a string and returns a boolean that indicates if a match is found or not.

New in version 4.2.

Replaces the first instance of a matched string in a given input.

New in version 4.4.

Replaces all instances of a matched string in a given input.

New in version 4.4.

Removes whitespace or the specified characters from the end of a string.

New in version 4.0.

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.
Returns the number of UTF-8 encoded bytes in a string.
Returns the number of UTF-8 code points in a string.
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.
Deprecated. Use $substrBytes or $substrCP.
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.
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.
Converts a string to lowercase. Accepts a single argument expression.

Converts value to a string.

New in version 4.0.

Removes whitespace or the specified characters from the beginning and end of a string.

New in version 4.0.

Converts a string to uppercase. Accepts a single argument expression.
Name
Description
Access available per-document metadata related to the aggregation operation.

Trigonometry expressions perform trigonometric operations on numbers. Values that represent angles are always input or output in radians. Use $degreesToRadians and $radiansToDegrees to convert between degree and radian measurements.

Name
Description
Returns the sine of a value that is measured in radians.
Returns the cosine of a value that is measured in radians.
Returns the tangent of a value that is measured in radians.
Returns the inverse sin (arc sine) of a value in radians.
Returns the inverse cosine (arc cosine) of a value in radians.
Returns the inverse tangent (arc tangent) of a value in radians.
Returns the inverse tangent (arc tangent) of y / x in radians, where y and x are the first and second values passed to the expression respectively.
Returns the inverse hyperbolic sine (hyperbolic arc sine) of a value in radians.
Returns the inverse hyperbolic cosine (hyperbolic arc cosine) of a value in radians.
Returns the inverse hyperbolic tangent (hyperbolic arc tangent) of a value in radians.
Returns the hyperbolic sine of a value that is measured in radians.
Returns the hyperbolic cosine of a value that is measured in radians.
Returns the hyperbolic tangent of a value that is measured in radians.
Converts a value from degrees to radians.
Converts a value from radians to degrees.
Name
Description

Converts a value to a specified type.

New in version 4.0.

Returns boolean true if the specified expression resolves to an integer, decimal, double, or long.

Returns boolean false if the expression resolves to any other BSON type, null, or a missing field.

New in version 4.4.

Converts value to a boolean.

New in version 4.0.

Converts value to a Date.

New in version 4.0.

Converts value to a Decimal128.

New in version 4.0.

Converts value to a double.

New in version 4.0.

Converts value to an integer.

New in version 4.0.

Converts value to a long.

New in version 4.0.

Converts value to an ObjectId.

New in version 4.0.

Converts value to a string.

New in version 4.0.

Return the BSON data type of the field.

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.

Name
Description
Returns the result of a user-defined accumulator function.
Returns an array of unique expression values for each group. Order of the array elements is undefined.
Returns an average of numerical values. Ignores non-numeric values.

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

Distinct from the $first array operator.

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

Distinct from the $last array operator.

Returns the highest expression value for each group.
Returns a document created by combining the input documents for each group.
Returns the lowest expression value for each group.
Returns an array of expression values for each group.
Returns the population standard deviation of the input values.
Returns the sample standard deviation of the input values.
Returns a sum of numerical values. Ignores non-numeric values.

Some operators that are available as accumulators for the $group stage are also available for use in other stages but not as accumulators. When used in these other stages, these operators do not maintain their state and can take as input either a single argument or multiple arguments. For details, refer to the specific operator page.

Changed in version 3.2.

The following accumulator operators are also available in the $project, $addFields, and $set stages.

Name
Description
Returns an average of the specified expression or list of expressions for each document. Ignores non-numeric values.
Returns the maximum of the specified expression or list of expressions for each document
Returns the minimum of the specified expression or list of expressions for each document
Returns the population standard deviation of the input values.
Returns the sample standard deviation of the input values.
Returns a sum of numerical values. Ignores non-numeric values.
Name
Description

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.

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