DATAJDBC-218 - Add support for key column in @Column annotation.

Original pull request: #83.

src/main/java/org/springframework/data/relational/core/mapping/BasicRelationalPersistentProperty.java

@@ -38,6 +38,7 @@ import org.springframework.util.ClassUtils;
  *
  * @author Jens Schauder
  * @author Greg Turnquist
+ * @author Florian Lüdiger
  */
 class BasicRelationalPersistentProperty extends AnnotationBasedPersistentProperty<RelationalPersistentProperty>
 		implements RelationalPersistentProperty {
@@ -52,6 +53,7 @@ class BasicRelationalPersistentProperty extends AnnotationBasedPersistentPropert
 
 	private final RelationalMappingContext context;
 	private final Lazy<Optional<String>> columnName;
+	private final Lazy<Optional<String>> keyColumnName;
 
 	/**
 	 * Creates a new {@link AnnotationBasedPersistentProperty}.
@@ -70,6 +72,8 @@ class BasicRelationalPersistentProperty extends AnnotationBasedPersistentPropert
 
 		this.context = context;
 		this.columnName = Lazy.of(() -> Optional.ofNullable(findAnnotation(Column.class)).map(Column::value));
+		this.keyColumnName = Lazy.of(() -> Optional.ofNullable(
+				findAnnotation(Column.class)).map(Column::keyColumn).filter(keyColumn -> !keyColumn.equals("")));
 	}
 
 	/*
@@ -116,7 +120,10 @@ class BasicRelationalPersistentProperty extends AnnotationBasedPersistentPropert
 
 	@Override
 	public String getKeyColumn() {
-		return isQualified() ? context.getNamingStrategy().getKeyColumn(this) : null;
+		if (isQualified())
+			return keyColumnName.get().orElseGet(() -> context.getNamingStrategy().getKeyColumn(this));
+		else
+			return null;
 	}
 
 	@Override

src/main/java/org/springframework/data/relational/core/mapping/Column.java

@@ -25,6 +25,7 @@ import java.lang.annotation.Target;
  * The annotation to configure the mapping from an attribute to a database column.
  *
  * @author Kazuki Shimizu
+ * @author Florian Lüdiger
  */
 @Retention(RetentionPolicy.RUNTIME)
 @Target({ ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE })
@@ -35,4 +36,9 @@ public @interface Column {
 	 * The mapping column name.
 	 */
 	String value();
+
+	/**
+	 * The column name for key columns of List or Map collections.
+	 */
+	String keyColumn() default "";
 }