code: public AtlasVertex getVertexForInstanceByUniqueAttribute(ClassType classType, IReferenceableInstance instance)
          throws AtlasException {
          if (LOG.isDebugEnabled()) {
              LOG.debug("Checking if there is an instance with the same unique attributes for instance {}", instance.toShortString());
          }
  
          AtlasVertex result = null;
          for (AttributeInfo attributeInfo : classType.fieldMapping().fields.values()) {
              if (attributeInfo.isUnique) {
                  String propertyKey = getQualifiedFieldName(classType, attributeInfo.name);
                  try {
                      result = findVertex(propertyKey, instance.get(attributeInfo.name),
                              Constants.ENTITY_TYPE_PROPERTY_KEY, classType.getName(),
                              Constants.STATE_PROPERTY_KEY, Id.EntityState.ACTIVE.name());
                      if (LOG.isDebugEnabled()) {
                          LOG.debug("Found vertex by unique attribute : {}={}", propertyKey, instance.get(attributeInfo.name));
                      }
                  } catch (EntityNotFoundException e) {
                      //Its ok if there is no entity with the same unique value
                  }
              }
          }
  
          return result;
      }
docs: For the given type, finds an unique attribute and checks if there is an existing instance with the same
  unique value
  
  @param classType
  @param instance
  @return
  @throws AtlasException