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Transform flattened fields to flat_object

Migration Assistant automatically converts Elasticsearch flattened fields into OpenSearch flat_object fields during metadata migration when the source contains flattened mappings and the target supports flat_object. If the target does not support the equivalent field behavior you need, plan a custom transformation instead.

Built-in transformation behavior

During metadata migration, built-in transformations detect flattened field definitions and rewrite them to flat_object automatically. No manual configuration is required.

Identifying flattened fields

To verify whether your source uses flattened fields, run the following command:

console clusters curl source /_mapping

If the response contains "type":"flattened", the automatic transformation applies during metadata migration.

Post-migration validation

After the metadata phase, verify that the target mappings are correct:

console clusters curl target /your-index/_mapping
workflow show

Also validate the following against the target:

• Representative queries
• Aggregations
• Dashboards or visualizations that depend on the field.

Custom transformer

Use a custom metadata transformer if:

• The target version does not support the field behavior you need.
• You want to convert flattened into a different target type.
• You need additional property cleanup beyond the built-in transformation.