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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-42137] Allow aggregation $merge stage to write to a collection that the query also reads from</title>
                <link>https://jira.mongodb.org/browse/SERVER-42137</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Enable functionality to permit $out to merge/append to the aggregation source collection.&lt;/p&gt;</description>
                <environment></environment>
        <key id="850104">SERVER-42137</key>
            <summary>Allow aggregation $merge stage to write to a collection that the query also reads from</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="mihai.andrei@mongodb.com">Mihai Andrei</assignee>
                                    <reporter username="clare.scally@mongodb.com">Clare Scally</reporter>
                        <labels>
                    </labels>
                <created>Wed, 10 Jul 2019 14:20:25 +0000</created>
                <updated>Sun, 29 Oct 2023 22:19:12 +0000</updated>
                            <resolved>Wed, 20 Nov 2019 20:46:13 +0000</resolved>
                                    <version>4.2.0-rc2</version>
                                    <fixVersion>4.3.2</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>1</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="2553777" author="xgen-internal-githook" created="Wed, 20 Nov 2019 15:40:50 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Mihai Andrei&apos;, &apos;email&apos;: &apos;mihai.andrei@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-42137&quot; title=&quot;Allow aggregation $merge stage to write to a collection that the query also reads from&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-42137&quot;&gt;&lt;del&gt;SERVER-42137&lt;/del&gt;&lt;/a&gt; Allow aggregation  stage to write to a collection that the query also reads from&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/b4f98455b02fa64dd23be3512ef83649a5395b76&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/b4f98455b02fa64dd23be3512ef83649a5395b76&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2327954" author="guy@southbanksoftware.com" created="Tue, 16 Jul 2019 02:24:03 +0000"  >&lt;p&gt;Yes, we duplicate multiple documents but only for a specific key value&#160;&lt;/p&gt;

&lt;p&gt;The prototoype pipeline we used in 4.1 went like this:&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;$match : select the documents to be cloned using a key value&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;$project: Remove the existing&#160; key&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;$addFields: Add the new key&#160;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;$out: append the new documents to the same collection&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;4.1 allowed this behavior with mode:&apos;insertDocuments&apos;.&#160;&lt;/p&gt;</comment>
                            <comment id="2327572" author="asya" created="Mon, 15 Jul 2019 19:56:40 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=guy%40southbanksoftware.com&quot; class=&quot;user-hover&quot; rel=&quot;guy@southbanksoftware.com&quot;&gt;guy@southbanksoftware.com&lt;/a&gt; I&apos;m trying to understand the use case - you say you are modifying the documents into the same collection, but yet you&apos;re using insert mode, so when you say you are cloning the document, does this mean when you start with ten documents, you expect to end up with twenty?  &lt;/p&gt;

&lt;p&gt;How do you generate a new unique &quot;merge on&quot; field? &lt;/p&gt;</comment>
                            <comment id="2324547" author="guy@southbanksoftware.com" created="Sun, 14 Jul 2019 01:54:09 +0000"  >&lt;p&gt;We have a use case in which we are cloning a set of documents into the same collection (with some modifications).&#160; We were enthusiastically awaiting the enhancements to $out that appeared in 4.1 which allowed us to do this cloning on the server side without a network round trip.&#160; &#160;It was disappointing to see when 4.2 arrived that the capability which existed in 4.1 had been removed.&#160;&lt;/p&gt;

&lt;p&gt;I do understand how in MongoDB there is a chance of an infinite loop if the $merge stage generates documents that might be read by the $match clause if there is one or even worse if there is no $match.&#160; &lt;/p&gt;

&lt;p&gt;In our use case, the $match clause specifically selects documents that cannot match those output by the pipeline - there&apos;s a $project and $addfields that modifies the output to prevent that from occurring.&#160;&lt;/p&gt;

&lt;p&gt;The functionality is essentially INSERT INTO t1 SELECT * FROM t1.&#160; In a relational database, a query snapshot would prevent the SELECT from reading anything created by the INSERT.&#160;&#160;&lt;/p&gt;

&lt;p&gt;So,&#160; we would really like to have this capability.&#160; Given what I understand of MongoDB architecture, it would seem that the easiest way to do this would be to append the output to a temporary collection until all earlier stages of the pipeline are complete and then append that output to the source collection.&#160;&lt;/p&gt;


&lt;p&gt;&#160;&lt;/p&gt;</comment>
                    </comments>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="611439">SERVER-37378</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="642013">SERVER-38360</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="1901236">SERVER-60788</issuekey>
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                            <customfieldvalue>This patch removes the following restrictions on $merge (both from &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/merge/#merge-into):&quot;&gt;https://docs.mongodb.com/manual/reference/operator/aggregation/merge/#merge-into):&lt;/a&gt;&lt;br/&gt;
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-The output collection cannot be the same collection as the collection being aggregated&lt;br/&gt;
-The output collection cannot appear in any other stages of the pipeline &lt;br/&gt;
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