How to put database inside a struct?

[rix0rrr]

Hi,

I'm trying to use native_db, but I have a feeling I'm doing something wrong...

I'd like to hide the concern of how my database is implemented, so I'd like to hide the implementation details in a struct with some methods. But the combination of DatabaseBuilder and Database which borrows the builder is making this hard.

My first attempt was this, but the lifetime I have to pass into the Database leads to a parameter that I don't actually want to give. I want to pass a lifetime like 'self or 'this or something, but that doesn't exist:

pub struct Database<'a> {
    //            ^^^ it's not actually a parameter
    builder: DatabaseBuilder,
    db: native_db::Database<'a>,
}

It also becomes nearly impossible to implement the initializer for this struct:

    pub fn new() -> Result<Self> {
        let mut builder = DatabaseBuilder::new();
        builder.define::<Task>()?;
        Ok(Self {
            builder,
            db: builder.open("db.db")?,
            // ^^^^ error[E0597]: `builder` does not live long enough
        })
    }

I was ultimately somewhat able to make it work by doing contortions, by putting the Database into an Option<> and initializing it after construction.

pub struct Database<'a> {
    builder: DatabaseBuilder,
    db: Option<native_db::Database<'a>>,
}

impl<'a> Database<'a> {
    pub fn new() -> Result<Self> {
        let mut ret = Self {
            builder: DatabaseBuilder::new(),
            db: None,
        };
        ret.builder.define::<Task>()?;
        Ok(ret)
    }

    pub fn open(&'a mut self) -> Result<()> {
        //      ^^^^ this is the magic
        self.db = Some(self.builder.open("db.db")?);
        Ok(())
    }
}

This doesn't feel ideal, and everywhere I want to accept the database now I'm still stuck with that annoying lifetime parameter.

Another thing I figured I could do is just store the builder, and open and close the database on every mutation. But, I don't know. It's not clear to me that how it was designed to be used, and it intuitively feels wasteful to do that.

So here's my question: how should I put a database connection inside a struct?

[vincent-herlemont]

@rix0rrr The issue you are encountering is attempting a self-reference in a struct, which is "possible" but very limited in Rust. In our case, it's not feasible because there is a custom drop in redb.

For a brief explanation: Redb is used in the backend and its definition, redb::TableDefinition, has a name: &'a str field which is a reference. This forces us to store it in an external variable outside the Database struct.

👉 So, I would advise against trying to do this.

Instead, try to ensure that DatabaseBuilder lives as long as Database.

Feel free to post your new design.

[Gawdl3y]

This is essentially the same issue that I'm grappling with in #69. The requirement to keep the DatabaseBuilder around for the same life as Database is less than intuitive, I'd say, and actually nearly impossible to do in stateful applications without some hacky-feeling workarounds.

It looks like the creator of redb intends on removing as many lifetimes as possible from the public API for 2.0, based on a checklist item in cberner/redb#677 (and some of it has happened in cberner/redb#732). Hopefully that comes to fruition, as the lifetimes are making it tough to work with.