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                                            Change By:
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                                        <td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0px; border-collapse: collapse; padding: 0 0 10px 0"> <span class="diffcontext">Today&nbsp;the&nbsp;key&nbsp;used&nbsp;is&nbsp;a&nbsp;{{PersistentEntityKey}}&nbsp;class&nbsp;(and&nbsp;its&nbsp;association&nbsp;and&nbsp;idsource&nbsp;equivalent</span> <span class="diffaddedchars" style="background-color:#ddfade;">)</span> <span class="diffcontext">.<br /><br />We&nbsp;could&nbsp;offer&nbsp;a&nbsp;few&nbsp;options&nbsp;to&nbsp;the&nbsp;user</span> <span class="diffaddedchars" style="background-color:#ddfade;">:</span> <span class="diffcontext"><br /><br />h3.&nbsp;Options<br /><br />h4.&nbsp;Map&nbsp;keys&nbsp;as&nbsp;(Linked)&nbsp;Maps<br /><br />Instead&nbsp;of&nbsp;a&nbsp;specific&nbsp;type,&nbsp;one&nbsp;could&nbsp;return&nbsp;a&nbsp;Map&lt;String,Object&gt;&nbsp;to&nbsp;represent&nbsp;the&nbsp;key.&nbsp;Which&nbsp;might&nbsp;be&nbsp;simpler&nbsp;to&nbsp;use&nbsp;outside&nbsp;of&nbsp;Hibernate&nbsp;OGM.<br />Note&nbsp;that&nbsp;for&nbsp;k/v&nbsp;that&nbsp;support&nbsp;externalizers,&nbsp;the&nbsp;persisted&nbsp;structure&nbsp;does&nbsp;not&nbsp;have&nbsp;to&nbsp;change&nbsp;necessarily.<br /><br />h4.&nbsp;Direct&nbsp;id&nbsp;for&nbsp;keys&nbsp;in&nbsp;single&nbsp;column&nbsp;cases<br /><br />When&nbsp;the&nbsp;id&nbsp;is&nbsp;made&nbsp;of&nbsp;a&nbsp;single&nbsp;column&nbsp;and&nbsp;of&nbsp;a&nbsp;basic&nbsp;type,&nbsp;we&nbsp;can&nbsp;use&nbsp;that&nbsp;id&nbsp;as&nbsp;the&nbsp;key&nbsp;directly&nbsp;without&nbsp;having&nbsp;to&nbsp;create&nbsp;a&nbsp;{{PersistenceEntityKey}}&nbsp;structure.<br />It&nbsp;makes&nbsp;for&nbsp;a&nbsp;more&nbsp;natural&nbsp;access&nbsp;than&nbsp;the&nbsp;ad-hoc&nbsp;OGM&nbsp;structure&nbsp;(or&nbsp;even&nbsp;a&nbsp;map).<br /><br />Note&nbsp;that&nbsp;for&nbsp;{{PersistentAssociationKey}}&nbsp;it&nbsp;would&nbsp;not&nbsp;work&nbsp;as&nbsp;this&nbsp;key&nbsp;structure&nbsp;is&nbsp;almost&nbsp;always&nbsp;complex&nbsp;(multiple&nbsp;columns&nbsp;like&nbsp;fk&nbsp;+&nbsp;index&nbsp;etc).<br /><br />h3.&nbsp;A&nbsp;discussion&nbsp;on&nbsp;key&nbsp;migration<br /><br />Storing&nbsp;the&nbsp;column&nbsp;names&nbsp;as&nbsp;part&nbsp;of&nbsp;the&nbsp;key&nbsp;can&nbsp;help&nbsp;during&nbsp;&quot;schema&quot;&nbsp;modifications&nbsp;but&nbsp;only&nbsp;in&nbsp;subset&nbsp;of&nbsp;them.&nbsp;<br />A&nbsp;migration&nbsp;happens&nbsp;in&nbsp;batch&nbsp;or&nbsp;on&nbsp;the&nbsp;fly&nbsp;and&nbsp;relies&nbsp;ont&nbsp;he&nbsp;ability&nbsp;to&nbsp;lookup&nbsp;with&nbsp;the&nbsp;new&nbsp;id&nbsp;and&nbsp;if&nbsp;that&nbsp;fails,&nbsp;lookup&nbsp;with&nbsp;the&nbsp;old&nbsp;id&nbsp;and&nbsp;do&nbsp;a&nbsp;migration&nbsp;before&nbsp;putting&nbsp;it&nbsp;back.<br /><br />There&nbsp;are&nbsp;several&nbsp;scenarios:<br /><br />*&nbsp;a&nbsp;new&nbsp;column&nbsp;is&nbsp;added&nbsp;(with&nbsp;a&nbsp;default&nbsp;value)&nbsp;or&nbsp;a&nbsp;column&nbsp;is&nbsp;removed:&nbsp;in&nbsp;this&nbsp;case,&nbsp;storing&nbsp;the&nbsp;column&nbsp;name&nbsp;does&nbsp;not&nbsp;help&nbsp;and&nbsp;a&nbsp;conversion&nbsp;/&nbsp;migration&nbsp;logic&nbsp;can&nbsp;be&nbsp;devised.<br />*&nbsp;reordering&nbsp;of&nbsp;columns.&nbsp;if&nbsp;two&nbsp;columns&nbsp;of&nbsp;the&nbsp;same&nbsp;type&nbsp;are&nbsp;reordered,&nbsp;then&nbsp;you&nbsp;need&nbsp;column&nbsp;names&nbsp;to&nbsp;differentiate&nbsp;the&nbsp;two&nbsp;between&nbsp;the&nbsp;old&nbsp;and&nbsp;new&nbsp;structure&nbsp;and&nbsp;to&nbsp;guarantee&nbsp;your&nbsp;equality&nbsp;comparison.<br />*&nbsp;when&nbsp;a&nbsp;type&nbsp;changes&nbsp;in&nbsp;one&nbsp;of&nbsp;the&nbsp;column,&nbsp;then&nbsp;you&nbsp;do&nbsp;not&nbsp;need&nbsp;the&nbsp;column&nbsp;name,&nbsp;the&nbsp;type&nbsp;will&nbsp;help&nbsp;during&nbsp;the&nbsp;equality&nbsp;comparison<br />*&nbsp;when&nbsp;the&nbsp;data&nbsp;in&nbsp;the&nbsp;id&nbsp;column&nbsp;change&nbsp;but&nbsp;neither&nbsp;its&nbsp;type&nbsp;nor&nbsp;its&nbsp;structure:&nbsp;then&nbsp;some&nbsp;id&nbsp;value&nbsp;conversion&nbsp;logic&nbsp;needs&nbsp;to&nbsp;be&nbsp;devised&nbsp;but&nbsp;the&nbsp;column&nbsp;name&nbsp;cannot&nbsp;help&nbsp;(say&nbsp;an&nbsp;id&nbsp;was&nbsp;the&nbsp;ssn&nbsp;123456789&nbsp;and&nbsp;is&nbsp;now&nbsp;the&nbsp;country-ssn&nbsp;FR-123456789).<br />*&nbsp;when&nbsp;the&nbsp;id&nbsp;value&nbsp;changes&nbsp;structure&nbsp;(same&nbsp;type)&nbsp;and&nbsp;the&nbsp;column&nbsp;name&nbsp;changes,&nbsp;then&nbsp;part&nbsp;of&nbsp;the&nbsp;logic&nbsp;in&nbsp;the&nbsp;previous&nbsp;point&nbsp;is&nbsp;optional&nbsp;and&nbsp;the&nbsp;column&nbsp;name&nbsp;can&nbsp;help.<br /><br />h3.&nbsp;Questions<br /><br />There&nbsp;is&nbsp;an&nbsp;agreement&nbsp;that&nbsp;using&nbsp;{{PersistentEntityKey}}&nbsp;by&nbsp;default&nbsp;is&nbsp;preferred&nbsp;over&nbsp;the&nbsp;{{Map}}&nbsp;structure.<br /><br />The&nbsp;remaining&nbsp;question&nbsp;is&nbsp;whether&nbsp;it&nbsp;is&nbsp;worth&nbsp;it&nbsp;to&nbsp;store&nbsp;the&nbsp;simple&nbsp;ids&nbsp;directly&nbsp;or&nbsp;via&nbsp;{{PersistentEntityKey}}.&nbsp;As&nbsp;the&nbsp;single&nbsp;benefit&nbsp;for&nbsp;migration&nbsp;is&nbsp;the&nbsp;case&nbsp;when&nbsp;the&nbsp;id&nbsp;value&nbsp;changes&nbsp;and&nbsp;the&nbsp;column&nbsp;name&nbsp;is&nbsp;changed&nbsp;(last&nbsp;point).<br /><br />h3.&nbsp;References<br /><br />The&nbsp;discussion&nbsp;happened&nbsp;in&nbsp;the&nbsp;following&nbsp;thread&nbsp;http://lists.jboss.org/pipermail/hibernate-dev/2014-November/011929.html&nbsp;and&nbsp;on&nbsp;IRC&nbsp;/&nbsp;Hangout</span> 
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