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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-37791] Excluding non-existent field in $project can adversely impact downstream pipeline stages</title>
                <link>https://jira.mongodb.org/browse/SERVER-37791</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;A &lt;tt&gt;$project&lt;/tt&gt; stage which excludes a field that does not exist in the input document naively &quot;removes&quot; that field from the output document. In practice, this has the effect of &lt;b&gt;adding&lt;/b&gt; a new field to the output with value &quot;missing&quot;. While this is usually invisible to the client, since the &quot;missing&quot; field will be discarded when the document is serialized to BSON, the unexpected addition of this new field can negatively impact the behaviour of downstream pipeline stages.&lt;/p&gt;

&lt;p&gt;In the example below, for instance, we exclude a non-existent field &lt;tt&gt;b&lt;/tt&gt; before attempting to insert new fields &lt;tt&gt;a,b,c&lt;/tt&gt; into the document via the &lt;tt&gt;$addFields&lt;/tt&gt; stage. Normally the new fields are added in the order they are specified, but because the exclusion has already spuriously added an empty field &lt;tt&gt;b&lt;/tt&gt;, the ordering of the fields in the output document changes to &lt;tt&gt;b,a,c&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;Inclusions of non-existent fields do not produce these &quot;missing&quot; entries, and therefore do not manifest this bug.&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;
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						&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;&amp;gt; db.testing.insert({_id: 1})&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;WriteResult({ &quot;nInserted&quot; : 1 })&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;&amp;nbsp;&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;&amp;gt; db.testing.aggregate([{$addFields: {a: &quot;1&quot;, b: &quot;2&quot;, c: &quot;3&quot;}}])&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;_id&quot; : 1, &quot;a&quot; : &quot;1&quot;, &quot;b&quot; : &quot;2&quot;, &quot;c&quot; : &quot;3&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&amp;nbsp;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&amp;gt; db.testing.aggregate([{$project: {b: 1}}, {$addFields: {a: &quot;1&quot;, b: &quot;2&quot;, c: &quot;3&quot;}}])&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;_id&quot; : 1, &quot;a&quot; : &quot;1&quot;, &quot;b&quot; : &quot;2&quot;, &quot;c&quot; : &quot;3&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&amp;nbsp;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&amp;gt; db.testing.aggregate([{$project: {b: 0}}, {$addFields: {a: &quot;1&quot;, b: &quot;2&quot;, c: &quot;3&quot;}}])&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;{ &quot;_id&quot; : 1, &quot;b&quot; : &quot;2&quot;, &quot;a&quot; : &quot;1&quot;, &quot;c&quot; : &quot;3&quot; }&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;</description>
                <environment></environment>
        <key id="625808">SERVER-37791</key>
            <summary>Excluding non-existent field in $project can adversely impact downstream pipeline stages</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="david.storch@mongodb.com">David Storch</assignee>
                                    <reporter username="bernard.gorman@mongodb.com">Bernard Gorman</reporter>
                        <labels>
                            <label>qexec-team</label>
                            <label>query-44-grooming</label>
                    </labels>
                <created>Sat, 27 Oct 2018 01:57:01 +0000</created>
                <updated>Sun, 29 Oct 2023 22:27:06 +0000</updated>
                            <resolved>Fri, 10 Jan 2020 18:23:16 +0000</resolved>
                                    <version>3.4.17</version>
                    <version>3.6.8</version>
                    <version>4.0.3</version>
                    <version>4.1.4</version>
                                    <fixVersion>4.3.3</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="2719002" author="xgen-internal-githook" created="Fri, 10 Jan 2020 17:47:57 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;David Storch&apos;, &apos;email&apos;: &apos;david.storch@mongodb.com&apos;, &apos;username&apos;: &apos;dstorch&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-37791&quot; title=&quot;Excluding non-existent field in $project can adversely impact downstream pipeline stages&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-37791&quot;&gt;&lt;del&gt;SERVER-37791&lt;/del&gt;&lt;/a&gt; Prevent exclusion of non-existent field from affecting field order.&lt;/p&gt;

&lt;p&gt;Exclusion of an existing field can still affect the field&lt;br/&gt;
order produced by subsequent $project or $addFields stages.&lt;br/&gt;
Changing this is left as future work.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/d6dad3428ca5773f8fcb7e75767e4817f7a185dd&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/d6dad3428ca5773f8fcb7e75767e4817f7a185dd&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2585595" author="bernard.gorman" created="Thu, 5 Dec 2019 12:00:25 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch&quot; class=&quot;user-hover&quot; rel=&quot;david.storch&quot;&gt;david.storch&lt;/a&gt;: I don&apos;t there&apos;s any particularly compelling reason to keep this around - the only reason we retained it at the time was to prevent a behavioural change slipping into what was otherwise a pure refactor of the projection system.&lt;/p&gt;</comment>
                            <comment id="2584313" author="david.storch" created="Wed, 4 Dec 2019 22:18:57 +0000"  >&lt;p&gt;This come up in the patch for &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-44334&quot; title=&quot;Make ProjectionExecutor faster&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-44334&quot;&gt;&lt;del&gt;SERVER-44334&lt;/del&gt;&lt;/a&gt;, so I&apos;m flagging for re-triage. Is there any good reason not to just delete the special code we have to preserve this behavior?&lt;/p&gt;</comment>
                    </comments>
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