anchor.diff:  When working with the MongoDB backend, you can specify the following settings:
   
   * the write concern for entities and associations using the +@WriteConcern+ annotation
  +* the read preference for entities and associations using the +@ReadPreference+ annotation
   * a strategy for storing associations using the +@AssociationStorage+ and +@AssociationDocumentStorage+ annotations
   (refer to <<ogm-mongodb-storage-prinicples>> to learn more about these options).
   
  
positive.code: This is useful in cases where the pre-defined configurations are not sufficient,
  e.g. if you want to ensure that writes are propagated to a specific number of replicas or given "tag set".
  Only takes effect if +hibernate.ogm.mongodb.write_concern+ is set to +CUSTOM+.
  
  For more information, please refer to the
  http://api.mongodb.org/java/current/com/mongodb/WriteConcern.html[official documentation].
  
negative.code: ----
  @Entity
  @WriteConcern(WriteConcernType.JOURNALED)
  @ReadPreference(ReadPreferenceType.PRIMARY_PREFERRED)
  @AssociationStorage(AssociationStorageType.ASSOCIATION_DOCUMENT)
  @AssociationDocumentStorage(AssociationDocumentType.COLLECTION_PER_ASSOCIATION)
  public class Zoo {
  