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<channel>
    <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-22384] Add $lookUpOne to retrieve single Object on result</title>
                <link>https://jira.mongodb.org/browse/SERVER-22384</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Now $lookup returns array of matched Objects, this new return single Object (as single Object field, not Array)&lt;/p&gt;</description>
                <environment></environment>
        <key id="261987">SERVER-22384</key>
            <summary>Add $lookUpOne to retrieve single Object on result</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="1" iconUrl="https://jira.mongodb.org/images/icons/statuses/open.png" description="">Open</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="joseaio">Jose Antonio Illescas Olmo</reporter>
                        <labels>
                    </labels>
                <created>Sat, 30 Jan 2016 18:54:10 +0000</created>
                <updated>Thu, 6 Apr 2023 15:39:07 +0000</updated>
                                            <version>3.2.1</version>
                                    <fixVersion>features we&amp;#39;re not sure of</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>5</votes>
                                    <watches>15</watches>
                                                                                                                <comments>
                            <comment id="2221316" author="asya" created="Mon, 22 Apr 2019 22:41:39 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=roman_lehnert&quot; class=&quot;user-hover&quot; rel=&quot;roman_lehnert&quot;&gt;roman_lehnert&lt;/a&gt;&#160; expressive $lookup (which allows specifying the full pipeline) allows you to add {&lt;tt&gt;$limit:1&lt;/tt&gt;} to the $lookup directly.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;</comment>
                            <comment id="2214794" author="roman.lehnert@givve.com" created="Tue, 16 Apr 2019 11:09:36 +0000"  >&lt;p&gt;If i do not misunderstand sth. there might be one more reason for introducing a $lookupOne:&#160;&lt;/p&gt;

&lt;p&gt;The server does only know the number of resulting docs of a $lookup, followed by $unwind after the execution. And thus, the optiminzer cannot change a &lt;span class=&quot;error&quot;&gt;&amp;#91;$lookup, $unwind, $limit&amp;#93;&lt;/span&gt; to &lt;span class=&quot;error&quot;&gt;&amp;#91;$limit, $lookup, $unwind&amp;#93;&lt;/span&gt;.&lt;/p&gt;

&lt;p&gt;But a &lt;span class=&quot;error&quot;&gt;&amp;#91;($lookup(limit: 1)), $unwind, $limit&amp;#93;&lt;/span&gt; could be optimized to &lt;span class=&quot;error&quot;&gt;&amp;#91;$limit, $unwind, $lookup(limit: 1)&amp;#93;&lt;/span&gt; if the optimizer can inspect the stage details.&#160;&lt;/p&gt;

&lt;p&gt;If the optimizer cannot inspect the stage details, but only the stagenames, then the followin can be optimized:&lt;/p&gt;

&lt;p&gt;&lt;span class=&quot;error&quot;&gt;&amp;#91;$lookupOne, $limit&amp;#93;&lt;/span&gt; to &lt;span class=&quot;error&quot;&gt;&amp;#91;$limit, $lookupOne&amp;#93;&lt;/span&gt;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;</comment>
                            <comment id="1820380" author="asya" created="Thu, 1 Mar 2018 20:48:47 +0000"  >&lt;p&gt;This was a bug in the original 3.6.0 release.  The fix has been backported to 3.6.3 via BACKPORT-1528 so now that it&apos;s available you should be able to use expressive lookup with $match and $limit (and $unwind) which will be as fast as anything $lookupOne would do.&lt;/p&gt;</comment>
                            <comment id="1818717" author="ilyamokin" created="Wed, 28 Feb 2018 16:48:44 +0000"  >&lt;p&gt;Asya Kamsky,&lt;br/&gt;
lookup pipeline + unwind is extremely slow in my case! Maybe because that suggests additional deep clone for the records or something like that.&lt;br/&gt;
Even simple lookup more fast because can use indexes from the second collection. If you use lookup pipeline + $match we can forget about indexes.&lt;/p&gt;

&lt;p&gt;So the $lookUpOne will be really useful feature.&lt;/p&gt;</comment>
                            <comment id="1818702" author="asya" created="Wed, 28 Feb 2018 16:37:40 +0000"  >&lt;p&gt;As of 3.6, &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#join-conditions-and-uncorrelated-sub-queries&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;more expressive $lookup&lt;/a&gt; allows putting {&lt;tt&gt;$limit:1&lt;/tt&gt;} into the &lt;tt&gt;from&lt;/tt&gt; pipeline to limit &lt;tt&gt;$lookup&lt;/tt&gt; to matching only one value, including {&lt;tt&gt;$unwind:&quot;$as&quot;&lt;/tt&gt;} immediately after &lt;tt&gt;$lookup&lt;/tt&gt; will perform equivalent of lookup one.&lt;/p&gt;</comment>
                            <comment id="1741188" author="asya" created="Mon, 4 Dec 2017 07:23:48 +0000"  >&lt;p&gt;Sounds like you just need to use $unwind option &quot;preserveNullAndEmptyArrays&quot;&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/index.html#pipe._S_unwind&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/index.html#pipe._S_unwind&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1740951" author="manuelbcd" created="Sun, 3 Dec 2017 10:55:35 +0000"  >&lt;p&gt;I&apos;m trying to create a view with lookup using $unwind to retrieve 1 object per relationship instead of an array. It works but the resultset is omitting objects which do not have a match.&lt;br/&gt;
I want to list all the objects, even the ones that do not have a matched relationship.&lt;br/&gt;
Wouldn&apos;t be  $lookupOne useful for this purpose?&lt;/p&gt;</comment>
                            <comment id="1605927" author="alice@gothcandy.com" created="Sun, 25 Jun 2017 07:06:35 +0000"  >&lt;p&gt;Or&#8230; yet another implicit behaviour could totally be a solution.  :|&lt;/p&gt;</comment>
                            <comment id="1605914" author="asya" created="Sun, 25 Jun 2017 04:02:46 +0000"  >&lt;blockquote&gt;&lt;p&gt;I see $lookupOne as a sortcut (or single step) of $lookup + $unwind&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;Note that when $lookup is immediately followed by $unwind in the pipeline, the execution of the pipeline will fold this into a single stage:&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;db.baz.aggregate([{$lookup:{from:&quot;foo&quot;,as:&quot;bar&quot;,localField:&quot;id&quot;,foreignField:&quot;_id&quot;}},{$unwind:&quot;$bar&quot;}],{explain:true})&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;{&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;	&quot;stages&quot; : [&lt;/span&gt;&lt;/pre&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;		{&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;			&quot;$cursor&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;query&quot; : {&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;&amp;nbsp;&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;				&quot;queryPlanner&quot; : {&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;					&quot;plannerVersion&quot; : 1,&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;					&quot;namespace&quot; : &quot;geo.baz&quot;,&lt;/span&gt;&lt;/pre&gt;
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			&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;					&quot;parsedQuery&quot; : {&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;&amp;nbsp;&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;
			&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;					&quot;winningPlan&quot; : {&lt;/span&gt;&lt;/pre&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;						&quot;stage&quot; : &quot;EOF&quot;&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;					},&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;					&quot;rejectedPlans&quot; : [ ]&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;		},&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;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;$lookup&quot; : {&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;				&quot;from&quot; : &quot;foo&quot;,&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;				&quot;as&quot; : &quot;bar&quot;,&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;</comment>
                            <comment id="1292767" author="alice@gothcandy.com" created="Mon, 13 Jun 2016 20:41:46 +0000"  >&lt;p&gt;Why add an entirely new pipeline stage instead of a flag to the existing one?&lt;/p&gt;</comment>
                            <comment id="1166256" author="joseaio" created="Mon, 8 Feb 2016 10:04:25 +0000"  >&lt;p&gt;No, I see $lookupOne as a sortcut (or single step) of $lookup + $unwind&lt;/p&gt;</comment>
                            <comment id="1166234" author="asya" created="Mon, 8 Feb 2016 09:27:41 +0000"  >&lt;p&gt;You can use $lookup already to return one object - just add $unwind which will get folded into $lookup and you have just one object.  Is there another reason for having another stage in addition to $lookup?&lt;/p&gt;</comment>
                            <comment id="1165804" author="joseaio" created="Sat, 6 Feb 2016 17:16:32 +0000"  >&lt;p&gt;On relation exists 2 types of relation: to-many, to-one&lt;br/&gt;
 &#183; $lookup return a list of objects (0..N) =&amp;gt; Children of Person &lt;br/&gt;
 &#183; $lookupOne return only one object (0..1) =&amp;gt; Parent of Person&lt;br/&gt;
Thanks&lt;/p&gt;</comment>
                            <comment id="1160291" author="asya" created="Mon, 1 Feb 2016 22:18:06 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=joseaio&quot; class=&quot;user-hover&quot; rel=&quot;joseaio&quot;&gt;joseaio&lt;/a&gt; could you explain please what the primary goal of this feature would be?&lt;/p&gt;

&lt;p&gt;It seems rather limiting to retrieve a single object without being able to specify which one (via $sort, $max, etc).&lt;/p&gt;
</comment>
                            <comment id="1159116" author="stennie" created="Mon, 1 Feb 2016 02:10:30 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=joseaio&quot; class=&quot;user-hover&quot; rel=&quot;joseaio&quot;&gt;joseaio&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Thank you for the feature suggestion. I&apos;ve marked this for consideration during the next planning round.&lt;/p&gt;

&lt;p&gt;Regards,&lt;br/&gt;
Stephen&lt;/p&gt;</comment>
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