driver-core/src/main/com/mongodb/operation/MapReduceToCollectionOperation.java
public MapReduceToCollectionOperation action(final String action) {
        notNull("action", action);
        isTrue("action must be one of: \"replace\", \"merge\", \"reduce\"", VALID_ACTIONS.contains(action));
        this.action = action;
        return this;
    }
public MapReduceToCollectionOperation action(final String action) {
        notNull("action", action);
        isTrue("action must be one of: \"replace\", \"merge\", \"reduce\"", VALID_ACTIONS.contains(action));
        this.action = action;
        return this;
    }
Sets the output action one of: "replace", "merge", "reduce"

@param action the output action
@return this
@mongodb.driver.manual reference/command/mapReduce/#output-to-a-collection-with-an-action output with an action