DatasetSplitter.split with explicit fieldNames copies only the first value of each multi-valued field into the split indexes

Finding LFV-681Area: classification / utils: DatasetSplitter (split, createNewDoc fieldNames branch)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-681

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You split an index into training, test and cross-validation indexes with DatasetSplitter.split and name the fields to copy, for example {id, text, cls}, where text has two stored values in each document. You expect each new document to carry both values of text, as it does when fieldNames is null ('all should be used'). Instead only the first value is copied, because the named-fields branch reads document.getField(name), which returns only the first value, so the second and later values are missing from the stored fields and from the postings of every output index, with no error.

The test shows that DatasetSplitter.split(..., "cls", "text") with an explicit field name copies only the first value of the multi-valued stored field text into the split index.

Expected vs actual

Expected: the training document stores both values [first, second]; actual (-ea and -da): [first] only, because createNewDoc copies document.getField(name) (DatasetSplitter.java:189).

Files

• src/org/apache/lucene/classification/utils/TestDatasetSplitterMultiValuedField.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
• Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
• How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
• Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/classification/build/libs/lucene-classification-$V.jar
CP=$CP:$LUCENE/lucene/grouping/build/libs/lucene-grouping-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.classification.utils.TestDatasetSplitterMultiValuedField

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.