DATAMONGO-1097 - Add support for $mul to Update.

We now support multiply on Update allowing to multiply the value of the given key by a multiplier.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/query/Update.java

@@ -308,6 +308,22 @@ public class Update {
 		return this;
 	}
 
+	/**
+	 * Multiply the value of given key by the given number.
+	 * 
+	 * @see http://docs.mongodb.org/manual/reference/operator/update/mul/
+	 * @param key must not be {@literal null}.
+	 * @param multiplier must not be {@literal null}.
+	 * @return
+	 * @since 1.7
+	 */
+	public Update multiply(String key, Number multiplier) {
+
+		Assert.notNull(multiplier, "Multiplier must not be 'null'.");
+		addMultiFieldOperation("$mul", key, multiplier.doubleValue());
+		return this;
+	}
+
 	public DBObject getUpdateObject() {
 
 		DBObject dbo = new BasicDBObject();

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/query/UpdateTests.java

@@ -420,4 +420,24 @@ public class UpdateTests {
 		Update update = new Update().addToSet("key", new DateTime());
 		assertThat(update.toString(), is(notNullValue()));
 	}
+
+	/**
+	 * @see DATAMONGO-1097
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void multiplyShouldThrowExceptionForNullMultiplier() {
+		new Update().multiply("key", null);
+	}
+
+	/**
+	 * @see DATAMONGO-1097
+	 */
+	@Test
+	public void multiplyShouldAddMultiplierAsItsDoubleValue() {
+
+		Update update = new Update().multiply("key", 10);
+
+		assertThat(update.getUpdateObject(), equalTo(new BasicDBObjectBuilder().add("$mul", new BasicDBObject("key", 10D))
+				.get()));
+	}
 }