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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>
    </build-info>


<item>
            <title>[SERVER-8088] $unwind of non-array should be allowed</title>
                <link>https://jira.mongodb.org/browse/SERVER-8088</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;This would effectively just output the existing field.&lt;/p&gt;

&lt;p/&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;&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.reg.aggregate({$match:{text:/ba/i}}, {$unwind:&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;$text&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;
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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;   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;/pre&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;   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;errmsg&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;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;exception: $unwind:  value at end of field path must be an array&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;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
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&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;This will be important for collections where some document fields may be arrays, but some may not.&lt;/p&gt;</description>
                <environment></environment>
        <key id="61096">SERVER-8088</key>
            <summary>$unwind of non-array should be allowed</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="9">Done</resolution>
                                        <assignee username="eliot">Eliot Horowitz</assignee>
                                    <reporter username="scotthernandez">Scott Hernandez</reporter>
                        <labels>
                    </labels>
                <created>Sun, 6 Jan 2013 15:16:42 +0000</created>
                <updated>Tue, 15 May 2018 02:07:16 +0000</updated>
                            <resolved>Wed, 11 Mar 2015 20:22:17 +0000</resolved>
                                                    <fixVersion>3.1.0</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>11</votes>
                                    <watches>17</watches>
                                                                                                                <comments>
                            <comment id="849487" author="xgen-internal-githook" created="Wed, 11 Mar 2015 21:44:49 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;erh&apos;, u&apos;name&apos;: u&apos;Eliot Horowitz&apos;, u&apos;email&apos;: u&apos;eliot@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-8088&quot; title=&quot;$unwind of non-array should be allowed&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-8088&quot;&gt;&lt;del&gt;SERVER-8088&lt;/del&gt;&lt;/a&gt;: $unwind of scalar should return 1 doc with scalar&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/f4d17dd81431f9724006c0837ccac44068971b1d&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/f4d17dd81431f9724006c0837ccac44068971b1d&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="848128" author="ldolle@amadeus.com" created="Tue, 10 Mar 2015 21:34:19 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=dan%4010gen.com&quot; class=&quot;user-hover&quot; rel=&quot;dan@10gen.com&quot;&gt;dan@10gen.com&lt;/a&gt;,&lt;br/&gt;
Are the &quot;other parts of the system that can error&quot; (e.g., $push - cf. Asya&apos;s comment) also impacted by this new behaviour or are we only dealing here with unwind?&lt;br/&gt;
Thanks anyway for increasing the severity and considering a quick fix.&lt;/p&gt;

&lt;p&gt;&amp;#8211; Laurent&lt;/p&gt;</comment>
                            <comment id="847916" author="dan@10gen.com" created="Tue, 10 Mar 2015 19:03:34 +0000"  >&lt;p&gt;null field, empty array, missing field should all currently emit no document, which is correct.&lt;br/&gt;
We will change agg to emit a single document containing the scalar value as requested.&lt;/p&gt;</comment>
                            <comment id="679272" author="asya" created="Fri, 1 Aug 2014 22:34:58 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=ldolle%40amadeus.com&quot; class=&quot;user-hover&quot; rel=&quot;ldolle@amadeus.com&quot;&gt;ldolle@amadeus.com&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;One possible workaround would be to catch the exception when a non-array is attempted to be unwound and then rewrite the query programmatically based on the error.&lt;/p&gt;

&lt;p&gt;I do want to point out that aggregation is not the only part of the system that can error when array vs. non-arrays are inconsistent through the collection - any array operation like &quot;$push&quot;, etc. will also give an error if the targeted field is not of the expected type, so this is not something introduced by aggregation framework.&lt;/p&gt;

&lt;p&gt;Possible upstream workaround would involve normalizing all fields which are allowed to be arrays to always be arrays only - but since I don&apos;t know where the data comes from I cannot say whether that&apos;s feasible.&lt;/p&gt;

&lt;p&gt;Asya&lt;/p&gt;</comment>
                            <comment id="677074" author="sfrenkiel" created="Wed, 30 Jul 2014 23:58:23 +0000"  >&lt;p&gt;Small world..I enjoyed the talk also.  Had never heard of mtools, but it&apos;s been helpful for us &lt;img class=&quot;emoticon&quot; src=&quot;https://jira.mongodb.org/images/icons/emoticons/smile.png&quot; height=&quot;16&quot; width=&quot;16&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/p&gt;

&lt;p&gt;We&apos;ll look into this workaround.&lt;/p&gt;</comment>
                            <comment id="676784" author="ldolle@amadeus.com" created="Wed, 30 Jul 2014 18:52:08 +0000"  >&lt;p&gt;Yes, workaround on applicative side is definitely identified. Problem is implementation, which would be even uglier, as we have a lot of &quot;optional&quot; arrays, which could themselves contain other optional arrays, which could... etc.&lt;br/&gt;
As our application is a query generator which can handle any unpredictable user input (read: which can $unwind pretty much any array), we would end up with a complex workaround needing the schema to be hard-coded just to be able to eventually recreate the missing optional array, and its parent missing optional array, and its... etc.&lt;/p&gt;

&lt;p&gt;Definitely possible, but with a boilerplate effect. Thanks anyway for the tip, we&apos;ll eventually give it a try if this ticket does not move much.&lt;/p&gt;

&lt;p&gt;Do you know if investigating the server code and trying to implement that toggle on our side would be something feasible?&lt;/p&gt;

&lt;p&gt;&amp;#8211;&lt;br/&gt;
Totally unrelated, but I have to take the opportunity: Asya, I really liked your &quot;Sherlock Holmes&quot;/investigation talk at MongoDB World last month.&lt;br/&gt;
Nice work... and entertaining too!&lt;/p&gt;</comment>
                            <comment id="674570" author="asya" created="Tue, 29 Jul 2014 06:44:52 +0000"  >&lt;p&gt;There &lt;b&gt;is&lt;/b&gt; a work-around (ugly one) for determining whether a field is an array or not in the pipeline - you can add a $project which will turn the field into an array if that field is not an array. &lt;/p&gt;

&lt;p&gt;I describe a related hack to determine type here:  &lt;a href=&quot;http://www.kamsky.org/stupid-tricks-with-mongodb/is-it-an-array&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://www.kamsky.org/stupid-tricks-with-mongodb/is-it-an-array&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Instead of using it for $size, if it&apos;s not an array you can use an almost equally ugly trick (which will be slower, but at least it&apos;s something that can functionally work until the feature is implemented):&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;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;// assume &quot;a&quot; is the field that may or may not contain an array&lt;/span&gt;&lt;/pre&gt;
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					&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;db.collection.aggregate(&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
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&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;Now it&apos;s safe to unwind.  If you had any documents that had no field a then it&apos;ll be an empty array here and that will lose the document during unwind - there is a trick to rectify that as well by testing if field is equal to [ ] and then putting [ null ] or some such in there (which can be done in the else of the second $project above).&lt;/p&gt;
</comment>
                            <comment id="669828" author="ldolle@amadeus.com" created="Thu, 24 Jul 2014 09:57:35 +0000"  >&lt;p&gt;Any update on Jon&apos;s comment (25Apr14)?&lt;br/&gt;
If discarding-document-when-unwinded-field-is-empty-or-missing is a pure implementation choice, we&apos;d like to be able to switch this feature off.&lt;/p&gt;</comment>
                            <comment id="563097" author="jon.rangel@10gen.com" created="Fri, 25 Apr 2014 17:02:22 +0000"  >&lt;p&gt;Can the scope of this ticket be extended to include support for passing though documents that contain an empty array or non-existent field?  There would need to be an option to toggle the behaviour of $unwind.&lt;/p&gt;

&lt;p&gt;There are use cases involving $unwind followed by $group on _id where you want to aggregate array elements but don&apos;t want to filter docs from the output simply because they didn&apos;t contain any array elements.&lt;/p&gt;</comment>
                            <comment id="467886" author="sfrenkiel" created="Mon, 9 Dec 2013 17:49:04 +0000"  >&lt;p&gt;In my situation, I allow my client apps to extend my stored docs with their own properties, but because of this behavior at query time I am forced to make them tell me if the property is arrayed or not.  So this would definitely be a help to me in simplifying my interface.&lt;/p&gt;</comment>
                            <comment id="446475" author="scotthernandez" created="Fri, 25 Oct 2013 00:31:00 +0000"  >&lt;p&gt;While the $push behavior is different one is about reading/reporting while the other is about writing. During the transition from a single element, to array, it would be nice for this to work on reporting side, using aggregation; it also matches how queries/indexes work conceptually.&lt;/p&gt;</comment>
                            <comment id="436673" author="redbeard0531" created="Mon, 7 Oct 2013 17:30:42 +0000"  >&lt;p&gt;I think current behaviour is correct. This mimics $push which errors on non-missing/non-array fields.&lt;/p&gt;</comment>
                            <comment id="274379" author="jemanso" created="Sun, 24 Feb 2013 07:58:52 +0000"  >&lt;p&gt;I agree with you ... I&apos;ve also tried to do that way and got the same error.&lt;/p&gt;</comment>
                    </comments>
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