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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-14872] Aggregation pipeline project expression operator to concatenate multiple arrays into one</title>
                <link>https://jira.mongodb.org/browse/SERVER-14872</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;I need to count the number of duplicate IDs that occur across all documents in a collection.  However, this very simple aggregation is complicated by the fact that the IDs in question may appear in multiple fields in the input document.&lt;/p&gt;

&lt;p&gt;Suppose I have input documents like the one below:&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;{&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;  a : [1, 2, 3],&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;&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;  b : [2, 4, 5]&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;   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;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;I&apos;d like to transform this to a document that looks like:&lt;/p&gt;

&lt;p/&gt;
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&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;{&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&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;  c : [1, 2, 3, 2, 4, 5]&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;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;From this point, it is straightforward to unwind and group on c to get to the desired result.&lt;/p&gt;

&lt;p&gt;The above transformation is similar to what would be achieved using the $setUnion operator, but here I do not want to filter out duplicate values.&lt;/p&gt;

&lt;p&gt;Therefore, this request is for an operator similar to $setUnion but which does not filter out duplicate values.&lt;/p&gt;</description>
                <environment></environment>
        <key id="152214">SERVER-14872</key>
            <summary>Aggregation pipeline project expression operator to concatenate multiple arrays into one</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="9">Done</resolution>
                                        <assignee username="charlie.swanson@mongodb.com">Charlie Swanson</assignee>
                                    <reporter username="jon.rangel@mongodb.com">Jon Rangel</reporter>
                        <labels>
                    </labels>
                <created>Tue, 12 Aug 2014 17:37:19 +0000</created>
                <updated>Fri, 5 Feb 2016 14:29:49 +0000</updated>
                            <resolved>Mon, 15 Jun 2015 17:41:16 +0000</resolved>
                                                    <fixVersion>3.1.5</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>1</votes>
                                    <watches>9</watches>
                                                                                                                <comments>
                            <comment id="1025225" author="asya" created="Wed, 9 Sep 2015 15:17:24 +0000"  >&lt;blockquote&gt;&lt;p&gt;will evaluate to null if any of the inputs are null&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;Just highlighting this behavior, and adding a note for documentation that if the quoted behavior is not desired (i.e. if you want &lt;tt&gt;null&lt;/tt&gt; concat with array field to be just equal to array, then you should add &lt;tt&gt;$ifNull&lt;/tt&gt;:&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;   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;{$project: { newArray:  {$concatArrays: [ {$ifNull: [&quot;$a&quot;, [] ] }, {$ifNull: [&quot;$b&quot;, [] ] } ] } } }&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;Above will always output newArray as array type and never as null, even if a and b are both null in the input documents.   If one of them is null then the output will be the other array.   If you want to guard against error if a or b may not be an array, you could add $isArray operator.&lt;/p&gt;</comment>
                            <comment id="940752" author="charlie.swanson" created="Mon, 15 Jun 2015 17:41:16 +0000"  >&lt;p&gt;The new operator is called &lt;tt&gt;$concatArrays&lt;/tt&gt;. It takes any number of arguments, will evaluate to null if any of the inputs are null, and errors if any arguments are not null or arrays. Nested arrays will not be flattened (i.e {&lt;tt&gt;$concatArrays: [[1,2], [3, [4]]]&lt;/tt&gt;} will evaluate to &lt;tt&gt;[1, 2, 3, [4]]&lt;/tt&gt;).&lt;/p&gt;

&lt;p&gt;See the test in the commit for more details.&lt;/p&gt;</comment>
                            <comment id="940742" author="xgen-internal-githook" created="Mon, 15 Jun 2015 17:35:35 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;cswanson310&apos;, u&apos;name&apos;: u&apos;Charlie Swanson&apos;, u&apos;email&apos;: u&apos;charlie.swanson@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-14872&quot; title=&quot;Aggregation pipeline project expression operator to concatenate multiple arrays into one&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-14872&quot;&gt;&lt;del&gt;SERVER-14872&lt;/del&gt;&lt;/a&gt;: Aggregation expression to concatenate multiple arrays into one&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/5717454bc3c50c9ac985dcb77639f2710dfda8e4&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/5717454bc3c50c9ac985dcb77639f2710dfda8e4&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="878127" author="jon.rangel@10gen.com" created="Fri, 10 Apr 2015 14:03:58 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=asya&quot; class=&quot;user-hover&quot; rel=&quot;asya&quot;&gt;asya&lt;/a&gt; - $setIntersection does not help with the use case in the original description since the requirement is to check for duplicate IDs globally across the collection, not just within a single document.  $setIntersection would throw away IDs that we actually want to keep for the subsequent $unwind and $group stages.&lt;/p&gt;

&lt;p&gt;Agree with proposal for array $concat operator.&lt;/p&gt;</comment>
                            <comment id="876300" author="asya" created="Wed, 8 Apr 2015 20:34:44 +0000"  >&lt;p&gt;While the use case described (of finding duplicates) can be handled with set operations, there is a need for a general way of concatenating arrays, so I&apos;d like to use this ticket for that request.&lt;/p&gt;

&lt;p&gt;Proposed: {&lt;tt&gt;$project:{c:{$concat:[&quot;$a&quot;,&quot;$b&quot;]&lt;/tt&gt;}}} would combine a and b arrays into array c.&lt;/p&gt;</comment>
                            <comment id="875919" author="asya" created="Wed, 8 Apr 2015 16:30:49 +0000"  >&lt;p&gt;Would $setIntersection help - it will basically show duplicates between a and b and $size will show how many are duplicated.&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10010">
                    <name>Duplicate</name>
                                                                <inwardlinks description="is duplicated by">
                                        <issuelink>
            <issuekey id="39543">SERVER-5919</issuekey>
        </issuelink>
                            </inwardlinks>
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                            <issuelinktype id="10012">
                    <name>Related</name>
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                                        <issuelink>
            <issuekey id="225137">CSHARP-1364</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="209231">DRIVERS-234</issuekey>
        </issuelink>
                            </outwardlinks>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="183725">SERVER-17258</issuekey>
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