Relational PDA design through seeds

Question

I have somewhat what seems to be a fairly simple question.

Lets assume we are building a Todo list where everything is public. And lets say a user will be able to create a new Task and that task will have subtasks.

So, For the tasks I can create an account to store all the tasks with the follwing seeds.

pub struct Task {
  pub id: u64;
  pub count: u32; // This is important to keep track of the count of sub tasks on a task
}

This will allow me to create a task account with seed [TASK, task.id, &[bump]] where TASK is const TASK = b"task". And

pub struct SubTask {
  pub task_id: u64;
  pub id: u64;
}

Gives me subtask with seeds [SUBTASK, task.id, sub_task.id, &[bump]]. So this allows me to fetch, all the tasks and the subtasks related to a specific task.

But I was curious, how would I improve this so that I can also fetch all the tasks/sub tasks related to a user irrelevant of market. i.e. give me all the sub tasks an account with pubkey has created.

Thank you.

Answer 1

You could achieve this by including the user's public key in the PDA.

First include user information in the data structures:

pub struct Task {
    pub creator: Pubkey,
    pub id: u64,
    pub count: u32,
}

pub struct SubTask {
    pub creator: Pubkey,
    pub task_id: u64,
    pub id: u64,
}

Now update the seeds to include the users public key, which ensures that all tasks and subtasks created by a user can be directly associated with them.

In your case, for task the seeds would be:

[b"task", creator.key().as_ref(), &task_id.to_le_bytes(), &[bump]]

& for the subtask it would be:

[b"subtask", creator.key().as_ref(), &task_id.to_le_bytes(), &subtask_id.to_le_bytes(), &[bump]]

Then you can just create an instruction to filter accounts based on the owner. This would look something like:

pub fn get_user_tasks(ctx: Context<GetUserTasks>, user: Pubkey) -> ProgramResult {
    let tasks: Vec<Task> = ctx.accounts
        .task
        .iter()
        .filter(|task| task.creator == user)
        .collect();
    
    // Similarly for SubTasks
    let sub_tasks: Vec<SubTask> = ctx.accounts
        .sub_task
        .iter()
        .filter(|sub_task| sub_task.creator == user)
        .collect();

    // Logic to handle this data or return it
    Ok(())
}