code: public Set<String> getRequiredColumnFamilies() {
      Set<String> set = new HashSet<String>();
      for (FieldMapping mapping : fieldMappings) {
        if (FieldMapping.MappingType.KEY != mapping.getMappingType())
        set.add(mapping.getFamilyAsString());
      }
      return set;
    }
docs: Get the column families required by this schema.
  
  @return The set of column families.