Programming pattern / architectural question

Question

I am currently working on a project where I have a BankAccount entity for some other entity.

Each bank account as a reference to a bank entity, an account number and opt

Answer 1

Where to put validation logic depends on what information needed to perform that validation. Validation should be performed within type that has enough information to do that. On the other hand validation logic complexity must be taken into account as well. For example, if you have Email data attached only to Person type it can validated "in place" within person type because validation is not complex (assuming only email format is checked) and Person is the only consumer. If on the other hand you have deal data (characterized by goods data and price data) consumed by Store and Garage (selling your old stuff) types with validation logic that goods must belong to the Deal initiator it makes sense to put validation inside Deal type.