public void setIdField(String idField) {
		this.idField = idField;
	}
Set the tuple field name which will be used as the Mongo _id. If not set, the MongoDriver will
will assign a ObjectId with a generated value.

@param idField the name of the field to use as the identity in MongoDB.
extensions/spring-xd-extension-mongodb/src/main/java/org/springframework/xd/mongodb/TupleWriteConverter.java