Tutorial : Database Migration Part II

Mapping tables

For all content imported, there is a mapping row that corresponds drupal's unique id to the original unique id of that content. These tables are very useful, when going back and tracking where the issue may have arrived from. Also, the mapping tables are useful when importing new data, to determine which nodes/terms/users not to re-import.

Depending on preference, each vocabulary or content type may be in its own mapping table, or if combining vocabularies and content types in the same mapping table, then the original table name is also stored.

The minimum columns in a mapping table needed are; an auto-increment primary key, drupal id (either nid, uid or tid), the old unique id. Other columns can be created as needed for quick reference and checking.

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Courtesy : Raincitystudios.com