Re: [Rails] Specify and validate requirements on an ActiveRecord association

On 2 Nov 2011, at 07:13, doug316 <doug316@gmail.com> wrote:

> Looking for some guidance/support on what feels like might not be an
> uncommon use-case.
>
> `User.has_many accounts`
> `User.has_many payment_methods`
> `Account.belongs_to :user`
> `Account.belongs_to :payment_method`
> `PaymentMethod.has_many :accounts`
>
> There are lots of `Accounts`. When I (a `User`) want to assign
> accounts to a `PaymentMethod`, I really don't want to scope against
> all of them, but rather only those that I own. While this can be done
> at the controller level, it feels like a data-level constraint.
>
> I will want a validation, to ensure I do not map `PaymentMethods` to
> `Accounts` with different `Users`. But I will also want a way to
> specify the scope of possible associations, effectively using the same
> logic as the validation. Placing conditions on the association doesn't
> restrict invalid associations, nor does it imply that.
>
> A useful syntax might be:
>
> `PaymentMethod.has_many :accounts, requirement: proc {self.user ==
> target.user}, scope: true, validate: true`
>
> Expressing `my_payment_method.accounts` would use `requirement` as a
> condition. A reflection on the association
> (`PaymentMethod.possible_accounts`) could return a relation that could
> be used to scope out the possible valid associations from the target
> (`Accounts`). The validate flag could generate a matching DRY
> validation that ensures the same condition at the instance level.
>
> Realizing that crossing realms of associations and validations might
> not be the most desirable, but seem inherently required in order to
> avoid repetition.
>
> Does anyone have any insight on this?

You could add a validation to account that checks that payment_method.user_id is the same as user_id.

Another approach would be to change your schema. Would this work?

user has_many payment_methods
user has_many accounts, :through => :payment_methods

Payment method is unchanged and for account you'd probably need has_one :through to get at the user

To me, the advantage of this method is that the case of an account having a payment_method from the wrong account becomes simply impossible to express.

Fred.

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