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DOCS-13883 add random number generator
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‎source/includes/extracts-agg-operators.yaml

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* - Name
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- Description
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* - :expression:`$rand`
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- Returns a random float between 0 and 1
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.. versionadded:: 4.4.2
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* - :expression:`$sampleRate`
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- Randomly select documents at a given rate. Although the exact
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number of documents selected varies on each run, the quantity
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chosen approximates the sample rate expressed as a percentage
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of the total number of documents.
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.. versionadded:: 4.4.2
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---
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ref: agg-operators-objects

‎source/reference/operator/aggregation.txt

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@@ -647,7 +647,13 @@ Alphabetical Listing of Expression Operators
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* - :expression:`$radiansToDegrees`
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- Converts a value from radians to degrees.
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* - :expression:`$rand`
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- Returns a random float between 0 and 1.
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.. versionadded:: 4.4.2
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* - :expression:`$range`
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chosen approximates the sample rate expressed as a percentage
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of the total number of documents.
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.. versionadded:: 4.4.2
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* - :expression:`$second`
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- Returns the seconds for a date as a number between 0 and 60

‎source/reference/operator/aggregation/rand.txt

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===================
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$rand (aggregation)
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===================
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.. default-domain:: mongodb
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.. contents:: On this page
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:local:
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:backlinks: none
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:depth: 1
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:class: singlecol
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Definition
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----------
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.. expression:: $rand
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.. versionadded:: 4.4.2
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Returns a random float between 0 and 1 each time it is called.
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:expression:`$rand` has the following syntax:
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.. code-block:: javascript
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{ $rand: {} }
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The :expression:`$rand` operator doesn't take any arguments.
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Behavior
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--------
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Each time ``$rand`` is called it will return a floating point value
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that has up to 17 digits after the decimal point. Trailing 0s are
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dropped so the actual number of digits may vary.
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Examples
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--------
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This code initializes a ``randomSamples`` collection with 100 documents
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that is used in the following examples.
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.. code-block:: javascript
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N = 100
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bulk = db.randomSamples.initializeUnorderedBulkOp()
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for ( i = 0; i < N; i++) { bulk.insert( {_id: i, random: 0 } ) }
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bulk.execute()
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Usage with Update Queries
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``$rand`` operator can be used with update query operations. In
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this example :method:`~db.collection.updateMany()` uses the ``$rand``
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operator to insert a different random number into each document
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in the ``randomSamples`` collection.
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.. code-block:: javascript
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db.randomSamples.updateMany(
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{},
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[
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{ $set: { "random": { $rand: {} } } }
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]
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)
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We can use :pipeline:`$project` to see the output. The
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:pipeline:`$limit` stage halts the pipeline after the third document.
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.. code-block:: javascript
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db.randomSamples.aggregate(
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[
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{ $project: {_id: 0, random: 1 } },
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{ $limit: 3 }
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]
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)
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The output shows the random values.
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.. code-block:: javascript
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:copyable: false
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{ "random" : 0.8751284485870464 }
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{ "random" : 0.515147067802108 }
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{ "random" : 0.3750004525681561 }
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Rounding to Control the Number of Output Digits
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you want a shorter random value, consider using :expression:`$round`.
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Note that the :pipeline:`$set` stage updates the document, if ``$rand``
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is called in a :pipeline:`$project` stage the underlying document is
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not modified.
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.. code-block:: javascript
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db.randomSamples.aggregate(
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[
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{ $match: {} },
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{ $set: { rounded: { $round: [ "$random", 4 ] } } },
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{ $out: "randomSamples" }
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]
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)
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The :pipeline:`$project` stage displays the original and rounded value
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for each document.
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.. code-block:: javascript
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db.randomSamples.aggregate(
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[
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{ $project: {_id:0, random:1, rounded: 1 } },
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{ $limit: 3 }
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]
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)
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The update documents look like this:
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.. code-block:: javascript
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:copyable: false
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{ "random" : 0.8751284485870464, "rounded" : 0.8751 }
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{ "random" : 0.515147067802108, "rounded" : 0.5151 }
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{ "random" : 0.3750004525681561, "rounded" : 0.375 }
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.. note::
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Like ``$rand``, the value returned by the ``$round`` operator does
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not include any trailing 0s so the number of digits returned may
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vary.
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Selecting Random Items From a Collection
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``$rand`` operator can be used in an aggregation pipeline to select
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random documents from a collection. In this example we use ``$rand`` to
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select about half the documents in the ``randomSamples`` collection.
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.. code-block:: javascript
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db.randomSamples.aggregate(
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[
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{ $match: { $expr: { $lt: [0.5, {$rand: {} } ] } } },
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{ $count: "numMatches" }
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]
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)
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There are 100 documents in ``randomSamples``. Running the sample code 5
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times produces the following output which approaches the expected value
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of 50 matches in a collection this size.
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.. code-block:: javascript
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:copyable: false
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{ "numMatches" : 49 }
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{ "numMatches" : 52 }
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{ "numMatches" : 54 }
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{ "numMatches" : 48 }
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{ "numMatches" : 59 }
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.. note::
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This example shows that the number of documents selected is
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different each time. If you need to select an exact number of
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documents, consider using :pipeline:`$sample` instead of ``$rand``.
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.. seealso::
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:query:`$rand (query) <$rand>`, :pipeline:`$sample`, :expression:`$round`
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‎source/reference/operator/aggregation/sample.txt

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)
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The operation returns three random documents.
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.. seealso::
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:expression:`$rand (aggregation) <$rand>`
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‎source/reference/operator/query-miscellaneous.txt

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=============================
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Miscellaneous Query Operators
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=============================
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.. default-domain:: mongodb
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.. contents:: On this page
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:local:
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:backlinks: none
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:depth: 1
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:class: singlecol
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.. include:: /includes/extracts/operators-toc-explanation.rst
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.. list-table::
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:widths: 25,75
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:header-rows: 1
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* - Name
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- Description
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* - :query:`$comment`
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- Adds a comment to a query predicate.
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* - :query:`$rand`
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- Generates a random float between 0 and 1.
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.. versionadded:: 4.4.2
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.. toctree::
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:titlesonly:
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:hidden:
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/reference/operator/query/comment
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/reference/operator/query/rand

‎source/reference/operator/query.txt

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:depth: 1
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:class: singlecol
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.. include:: /includes/extracts/operators-toc-explanation.rst
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.. _query-selectors:
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.. only:: website
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.. include:: /includes/fact-comparison-order.rst
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.. list-table::
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:widths: 30,70
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/reference/operator/query-bitwise
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Comments
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~~~~~~~~
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.. _query-projection-operators:
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Projection Operators
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--------------------
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.. only:: website
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.. list-table::
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:widths: 25,75
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* - Name
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* - :query:`$comment`
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* - :projection:`$`
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- Adds a comment to a query predicate.
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- Projects the first element in an array that matches the query condition.
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* - :projection:`$elemMatch`
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- Projects the first element in an array that matches the specified :projection:`$elemMatch` condition.
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* - :projection:`$meta`
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- Projects the document's score assigned during :query:`$text` operation.
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* - :projection:`$slice`
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- Limits the number of elements projected from an array. Supports skip and limit slices.
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.. toctree::
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:titlesonly:
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:hidden:
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/reference/operator/query/comment
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/reference/operator/projection
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.. _query-projection-operators:
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.. _query-miscelaneous-operators:
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Projection Operators
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--------------------
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Miscellaneous Operators
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-----------------------
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.. only:: website
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:widths: 30,70
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:widths: 25,75
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* - Name
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* - :projection:`$`
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- Projects the first element in an array that matches the query condition.
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* - :projection:`$elemMatch`
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- Projects the first element in an array that matches the specified :projection:`$elemMatch` condition.
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* - :projection:`$meta`
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- Projects the document's score assigned during :query:`$text` operation.
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* - :query:`$comment`
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* - :projection:`$slice`
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- Adds a comment to a query predicate.
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- Limits the number of elements projected from an array. Supports skip and limit slices.
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* - :query:`$rand`
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- Generates a random float between 0 and 1.
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.. toctree::
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:titlesonly:
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:hidden:
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/reference/operator/projection
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/reference/operator/query-miscellaneous
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