Immediate destination

The immediate destination is the ACH file-header field that identifies the receiving financial institution by routing number. During file processing, it should match one of the bank’s known routing numbers. A mismatch can indicate that a file belongs to another institution, creating privacy, disclosure, and regulatory risks—especially in multitenant systems. Because mergers and acquisitions can change a bank’s routing numbers, validation should use a maintained list rather than a permanently hardcoded value. section 8.3.1

A practical validation workflow includes:

• Maintain a database table containing routing numbers recognized as belonging to the bank. section 8.3.1 section 8.5.3
• Validate the immediate destination against that runtime list with a Pydantic field_validator. section 8.5.3
• Raise a specific error when the value is not an allowed routing number, and a separate error when validation cannot be completed. section 8.5.3
• Account for performance, although one ACH file normally has only one file-header record to validate. section 8.5.3

@field_validator('immediate_destination') #1
@classmethod    #2
def validate_immediate_destination(cls, v):
   try:
      if BankRoutingSql().valid_bank_routing_number(v): #2
         return v #3
      else:  #3
         raise ValueError('immediate_destination #3
➥ is not a valid routing number')  #3
   except Exception: #4
      raise ValueError('Unable to validate #4
➥ immediate_destination')  #4

The validator listing shows the implementation pattern: inspect the field, query the bank-routing service, return valid values, and convert failures into validation errors. section 8.5.3