# External Mapping of Column/Property in Data Class

**URL:** <https://forums.pega.com/t/external-mapping-of-column-property-in-data-class/5402>\
**Category:** General\
**Tags:** 8-8-3, pega-platform\
**Created:** [November 16, 2023, 12:45am UTC](https://forums.pega.com/t/external-mapping-of-column-property-in-data-class/5402 "2023-11-16T00:45:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![AbhishekC1725](https://avatars.discourse-cdn.com/v4/letter/a/9d8465/32.png) [@AbhishekC1725](https://forums.pega.com/u/AbhishekC1725)\
**Post date:** [November 16, 2023, 12:45am UTC](https://forums.pega.com/t/external-mapping-of-column-property-in-data-class/5402/1 "2023-11-16T00:45:25Z")

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How to do external mapping of column that we manually created in database by running query for creating column in table to the class present in pega.

Can someone please share the step-by-step configuration.

Thanks in advance.

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**Author:** ![ArulDevan](https://yyz2.discourse-cdn.com/pega/user_avatar/forums.pega.com/aruldevan/32/23602_2.png) [@ArulDevan](https://forums.pega.com/u/ArulDevan)\
**Post date:** [November 16, 2023, 12:57am UTC](https://forums.pega.com/t/external-mapping-of-column-property-in-data-class/5402/2 "2023-11-16T00:57:17Z")

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Hi @AbhishekC1725: If you have created the column names as same as the property names, it should work automatically. Please try by saving the class and database table instance. You can also do the mapping in the instance. Open the class, and navigate to external mapping tab to do the mapping. Please see [this](https://docs.pega.com/bundle/platform/page/platform/app-dev/external-mapping-tab-class-form.html) for details.

Thanks.

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<div class="post-metadata">

**Author:** ![AbhishekC1725](https://avatars.discourse-cdn.com/v4/letter/a/9d8465/32.png) [@AbhishekC1725](https://forums.pega.com/u/AbhishekC1725)\
**Post date:** [November 16, 2023, 5:22am UTC](https://forums.pega.com/t/external-mapping-of-column-property-in-data-class/5402/3 "2023-11-16T05:22:27Z")

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@ArulDevan

Hi,

Just want to know, that old data will come in the column after saving the class and database table instance.  
Or do we need to write activity to save all instances of that table?

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<div class="post-metadata">

**Author:** ![ArulDevan](https://yyz2.discourse-cdn.com/pega/user_avatar/forums.pega.com/aruldevan/32/23602_2.png) [@ArulDevan](https://forums.pega.com/u/ArulDevan)\
**Post date:** [November 16, 2023, 7:38am UTC](https://forums.pega.com/t/external-mapping-of-column-property-in-data-class/5402/4 "2023-11-16T07:38:30Z")

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Hi @AbhishekC1725: No. To get the data from the BLOB to the exposed columns, you may need to use column population (or) utilities (or) it will get exposed to the columns when there is a next write operation to the specific row.

Thanks.