How to use init_if_needed?

Question

I want to create an instruction where it is only possible to initialize a new PDA account if some parameter on another PDA account (same type) is fulfilled. So I was thinking that init_if_needed would be the best option for such an use-case.

In the case that the old account does exist, would I still be able to match on the correct bump when using init_if_needed - referring to old_acc:

    #[account(
        init_if_needed
        seeds = [seeds],
        bump = acc.bump,
        payer = payer
        space = SPACE
    )]
    pub old_acc
    
    #[account(
        init
        seeds = [seeds],
        bump,
        payer = payer
        space = SPACE
    )]
    pub new_acc

In the normal init i would just pass in "bump" without any assignment. Although not sure if I could still check for the correct bump if the account already exists.

So the question is how to account for the bumb in the case of account existence and non-existence

Answer 1

You should just omit the bump assertion (bump, instead of bump = ..., and let the Anchor generated code manage the calculation of it. There's no way to determine whether the bump assertion should exist based on account existence.