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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-41564] Introduce a $replaceFields pipeline stage</title>
                <link>https://jira.mongodb.org/browse/SERVER-41564</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The &lt;tt&gt;$addFields&lt;/tt&gt; pipeline stage allows us to replace a field if the field already exists.&#160; Unfortunately, the intention to &lt;em&gt;replace&lt;/em&gt; rather than &lt;em&gt;add&lt;/em&gt; becomes muddled for documents returned from the query that don&apos;t contain the field.&#160; The nonexistence of the field in those documents changes the effect from &lt;em&gt;replace&lt;/em&gt; to &lt;em&gt;add&lt;/em&gt;, even though we really just wanted to &lt;em&gt;replace&lt;/em&gt;.&lt;/p&gt;

&lt;p&gt;In my application, I often must return query results where nested arrays are filtered.&#160; I&apos;ve found that using&#160;&lt;tt&gt;$addFields&lt;/tt&gt;&#160;pipeline stages mostly accomplishes this.&#160; For each nested array that I must filter, I construct the corresponding pipeline stage as follows:&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: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;{ $addFields: { &apos;&quot;&lt;/span&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; + arrayFilter.field +&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;    &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;&apos;: { $filter: { input: &apos;$&quot;&lt;/span&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; + arrayFilter.field +&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;    &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;&apos;, cond: { &quot;&lt;/span&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; + arrayFilter.cond + &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot; } } } } }&quot;&lt;/span&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; );&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;But a problem arises when&#160;one of the path segments in&#160;&lt;tt&gt;arrayFilter.field&lt;/tt&gt;&#160;doesn&apos;t exist in the database.&#160; The problem is compounded when&#160;&lt;tt&gt;arrayFilter.field&lt;/tt&gt;&#160;is nested, i.e. contains dots.&lt;/p&gt;

&lt;p&gt;Under those circumstances, I want the stage to have zero effect. Unfortunately, the query result is such that:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;the path leading to the nonexistent array is materialized&lt;/li&gt;
	&lt;li&gt;the unwanted array is given a&#160;&lt;tt&gt;null&lt;/tt&gt;&#160;value.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;Such problems could be avoided by introducing a &lt;tt&gt;$replaceFields&lt;/tt&gt; pipeline stage that we can use to unambiguously declare our intention to &lt;em&gt;replace&lt;/em&gt; rather than &lt;em&gt;add&lt;/em&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="791726">SERVER-41564</key>
            <summary>Introduce a $replaceFields pipeline stage</summary>
                <type id="2" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14711&amp;avatarType=issuetype">New Feature</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="happynomad121@gmail.com">Adrian Pinter</reporter>
                        <labels>
                            <label>expression</label>
                    </labels>
                <created>Thu, 6 Jun 2019 17:02:36 +0000</created>
                <updated>Tue, 6 Dec 2022 02:57:35 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="2278499" author="eric.sedor" created="Mon, 10 Jun 2019 20:45:18 +0000"  >&lt;p&gt;Thanks for the added detail, &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=HappyNomad&quot; class=&quot;user-hover&quot; rel=&quot;HappyNomad&quot;&gt;HappyNomad&lt;/a&gt;; I&apos;m passing this on to an appropriate team for consideration. Please keep an eye on this ticket in case we have more questions or additional information about currently available workarounds.&lt;/p&gt;</comment>
                            <comment id="2277917" author="JIRAUSER1258097" created="Mon, 10 Jun 2019 16:38:51 +0000"  >&lt;p&gt;As an alternative to the&#160;&lt;tt&gt;$replaceFields&lt;/tt&gt;&#160;feature idea, you could consider changing&#160;&lt;tt&gt;$$REMOVE&lt;/tt&gt;&apos;s behavior in situations like this.&#160; It seems unhelpful to remove only the leaf field, while leaving behind the trail of empty documents that lead to it.&#160; &lt;tt&gt;$$REMOVE&lt;/tt&gt; could instead remove (or never include in the first place) every document in the path that doesn&apos;t exist in the database.&lt;/p&gt;</comment>
                            <comment id="2276891" author="JIRAUSER1258097" created="Sat, 8 Jun 2019 17:18:49 +0000"  >&lt;p&gt;Hi @eric.sedor, thanks for the doc links.&#160; Including computed fields was what I was already doing, but the &quot;conditionally exclude fields&quot; link led me to try something new:&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: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;{ $addFields: { &apos;&quot;&lt;/span&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; + arrayFilter.field +&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;	&lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;&apos;: { $ifNull: [ { $filter: { input: &apos;$&quot;&lt;/span&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; + arrayFilter.field +&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;	&lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;&apos;, cond: { &quot;&lt;/span&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; + arrayFilter.cond + &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot; } } }, &apos;$$REMOVE&apos; ] } } }&quot;&lt;/span&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; );&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;That got me a little closer, but no cigar.&#160; The problem remains when &lt;tt&gt;arrayFilter.field&lt;/tt&gt; is nested, i.e. contains dots.&#160; In that case, the path to the nonexistent array is still materialized in the query results.&#160; The only improvement over my earlier attempt is that the leaf array field is now absent rather than present with a &lt;tt&gt;null&lt;/tt&gt; value.&lt;/p&gt;</comment>
                            <comment id="2275051" author="eric.sedor" created="Thu, 6 Jun 2019 20:43:41 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=HappyNomad&quot; class=&quot;user-hover&quot; rel=&quot;HappyNomad&quot;&gt;HappyNomad&lt;/a&gt;; Do the $project options to &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/project/#conditionally-exclude-fields&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;conditionally exclude fields&lt;/a&gt; and &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/project/#include-computed-fields&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;include computed fields&lt;/a&gt; help satisfy your use-case?&lt;/p&gt;</comment>
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