Improved error message on missing back reference.

Closes #833
Original pull request 1384

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/SqlGenerator.java

@@ -260,7 +260,10 @@ class SqlGenerator {
 
 		Condition condition = null;
 		for (SqlIdentifier backReferenceColumn : parentIdentifier.toMap().keySet()) {
-
+			if (SqlIdentifier.EMPTY.equals(backReferenceColumn)){
+				throw new UnsupportedOperationException(
+						"An empty SqlIdentifier can't be used in condition. Make sure that all composite primary keys are defined in the query.");
+			}
 			Condition newCondition = table.column(backReferenceColumn).isEqualTo(getBindMarker(backReferenceColumn));
 			condition = condition == null ? newCondition : condition.and(newCondition);
 		}