retain does not work, if you only want to keep certain subfields

[TobiasNx]

Retain works with root level parent fields but not with an subfield:
https://github.com/TobiasNx/fix-FunctionalReview-Testing/blob/c9d9447983fc435d01b35ac57046dc1dd3ddbb11/data/testing/retainOnlySubfield

copy_field("node.properties.ccm:educationallearningresourcetype[]", "lrt[]")
retain("node.properties.ccm:educationallearningresourcetype[]", "lrt[]")

is:

{
  "lrt" : [ "https://w3id.org/kim/hcrt/image" ]
}

expected:

{
  "lrt" : [ "https://w3id.org/kim/hcrt/image" ],
  "node": {
    "properties": {
        "ccm:educationallearningresourcetype": [
            "https://w3id.org/kim/hcrt/image"
        ]
    }
}
}

[TobiasNx]

retain would be a clever element to learn fix-path since you would only keep certain elements.

would be a nice feature though.

[blackwinter]

Integration test:

• retainCertainSubfields

[blackwinter]

@TobiasNx: Implemented in #329. Please test.

[TobiasNx]

#329 (comment)