vantage-sql 0.10.0

Vantage extension for SQL databases (Postgres, MySQL, SQLite)
Documentation 
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//! `defer` for the dialects whose deferred query answers with one value.

use ciborium::Value as CborValue;
use vantage_expressions::traits::expressive::DeferredFn;
use vantage_expressions::{ExprDataSource, Expression};

use super::SqlDialect;

/// `expr` run on `db` when the deferred value is needed, answering with the
/// first column of the first row (untyped), or with every row when there is
/// no such cell.
pub(crate) fn defer_first_cell<D>(db: &D, expr: Expression<D::Value>) -> DeferredFn<D::Value>
where
    D: SqlDialect + ExprDataSource<D::Value> + Clone + Send + Sync + 'static,
{
    let db = db.clone();
    DeferredFn::from_fn(move || {
        let db = db.clone();
        let expr = expr.clone();
        Box::pin(async move { Ok(first_cell::<D>(db.execute(&expr).await?)) })
    })
}

/// The first column of the first row of a row array (untyped), or the rows
/// unchanged when there is no such cell.
pub(crate) fn first_cell<D: SqlDialect>(rows: D::Value) -> D::Value {
    match D::into_cbor(rows) {
        CborValue::Array(arr) => {
            let cell = match arr.first() {
                Some(CborValue::Map(map)) => map.first().map(|(_, v)| v.clone()),
                _ => None,
            };
            match cell {
                Some(v) => D::untyped(v),
                None => D::from_cbor_rows(CborValue::Array(arr)),
            }
        }
        other => D::from_cbor_rows(other),
    }
}