Apache NiFi RecordPath Guide
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With the above examples and explanation, we are able to easily reference a specific field within a Record. However, in real scenarios, the data is rarely as simple as in the examples above. Often times, we need to filter out or refine which fields we are referencing. Examples of when we might want to do this are when we reference an Array field and want to only reference some of the elements in the array; when we reference a Map field and want to reference one or a few specific entries in the Map; or when we want to reference a Record only if it adheres to some criteria. We can accomplish this by providing our criteria to the RecordPath within square brackets (using the [ and ] characters). We will go over each of these cases below.