Type projection and partial RETURNING result shapes

[alexeichhorn]

Context

This is a lower-priority architectural limitation.

Problem

Projection expressions are not connected to the result model's field types:

#[derive(dbkit::sqlx::FromRow)]
struct ResultRow {
    note: String,
}

let rows: Vec<ResultRow> = NullableRow::query()
    .select_only()
    .column_as(NullableRow::note, "note") // Option<String>
    .into_model()
    .all(&db)
    .await?;

This compiles and fails only if SQLx decodes a NULL value.

Partial mutation results have a related problem. Insert<FullModel> and Update<FullModel> retain Out = FullModel after .returning(vec![...]), even if only one column is returned. Executing the query then attempts to decode a partial row as the complete model.

Desired direction

• Make selected/returned result shapes explicit in the type system where practical.
• At minimum, partial RETURNING should not continue to claim that it returns the full model.
• If arbitrary projections remain an explicit SQLx decoding boundary, make that boundary clear in the API and documentation.
• Preserve an ergonomic escape hatch for custom FromRow projection types.

The implementation direction needs design work; this issue tracks the contract rather than prescribing a large type-level query builder.