code: def validateField(cls, fieldName, value) :
          
          try :
              valValue = Collection.validateField(fieldName, value)
          except SchemaViolation as e:
              if fieldName == "_from" or fieldName == "_to" :
                  return True
              else :
                  raise e
          return valValue
docs: checks if 'value' is valid for field 'fieldName'. If the validation is unsuccessful, raises a SchemaViolation or a ValidationError.
          for nested dicts ex: {address : { street: xxx} }, fieldName can take the form address.street