I would implement sensible default field mappings in the code,
which you can then override through the UI.
On the migration detail View tab, under the Mapping vertical tabs,
mappings that are pulled from the database (most likely, but not
necessarily, because they were overridden through the UI) are
The ability to export mappings to code most likely won't happen
until Migrate V3 on Drupal 8 - see https://drupal.org/node/1983404
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