code: public TableWriteItems addPrimaryKeyToDelete(PrimaryKey primaryKey) {
          if (primaryKey != null) {
              if (primaryKeysToDelete == null)
                  primaryKeysToDelete = new ArrayList<PrimaryKey>();
              checkConsistency(primaryKey);
              this.primaryKeysToDelete.add(primaryKey);
          }
          return this;
      }
docs: Adds a primary key to be deleted in a batch write-item operation. A
  primary key could consist of either a hash-key or both a
  hash-key and a range-key depending on the schema of the table.