add warning for field-based model with only 1 field

modules/core/src/main/java/org/terrier/matching/models/PerFieldNormWeightingModel.java

@@ -56,6 +56,7 @@
 public class PerFieldNormWeightingModel extends WeightingModel {
 
 	private static final long serialVersionUID = 1L;
+	protected static final Logger logger = LoggerFactory.getLogger(PerFieldNormWeightingModel.class);
 
 	protected String[] params;
 	protected BasicModel basicModel;
@@ -149,7 +150,10 @@ public void setCollectionStatistics(CollectionStatistics _cs)
 		super.setCollectionStatistics(_cs);
 		fieldCount = _cs.getNumberOfFields();
 		WeightingModelLibrary.checkForFields(_cs);
-
+		if (fieldCount == 1) {
+			logger.warn("Weighting model " + this.getClass().getSimpleName() + " is invoked on an index with only 1 field - this is probably incorrect.");
+		}
+
 		basicModel.setNumberOfDocuments(_cs.getNumberOfDocuments());
 		basicModel.setNumberOfTokens(_cs.getNumberOfTokens());