AR validation question

What is the best way to implement a validation for a withdrawal operation? So there is an Operation and Client models.
Client has_many :operations. In case of withdrawal operation, I’d like to check if the requested sum is not greater than a Client account balance. How to link the validation in the Operation model to a specific client ?

class Client < AR
has_many :operations
end

class Operation < AR
belongs_to :client
attr_accessible :sum
end

The routes are defined so that an operation could only created in the context of a Client.

Thanks

What is the best way to implement a validation for a withdrawal operation?
So there is an Operation and Client models.
Client has_many :operations. In case of withdrawal operation, I'd like to
check if the requested sum is not greater than a Client account balance. How
to link the validation in the Operation model to a specific client ?

In the custom validation of the operation you can use self.client to
give you its client.

Colin