Upsert (ON CONFLICT)

Make inserts idempotent or merge on key collisions in DuckDB through Diesel: on_conflict with do_nothing and do_update, and pulling proposed values from the excluded row.

DuckDB supports INSERT … ON CONFLICT, and the Diesel backend exposes it through .on_conflict(...). Use it to make an insert idempotent (DO NOTHING) or to merge new data onto an existing row (DO UPDATE). Verified in the crate’s tests/crud_dsl.rs.

DO NOTHING — idempotent insert

Skip the row silently when it would collide on the given target column(s):

use diesel::prelude::*;

diesel::insert_into(products::table)
    .values((products::id.eq(1), products::name.eq("dup"), products::price.eq(0.0)))
    .on_conflict(products::id)
    .do_nothing()
    .execute(&mut conn)?;

The affected-row count is 0 when the conflict caused the row to be skipped.

DO UPDATE — merge on conflict

On a collision, update the existing row. The excluded(...) helper references the proposed row (the values that would have been inserted):

use diesel::upsert::excluded;

diesel::insert_into(products::table)
    .values((products::id.eq(1), products::name.eq("replacement"), products::price.eq(30.0)))
    .on_conflict(products::id)
    .do_update()
    .set((
        products::name.eq(excluded(products::name)),
        products::price.eq(excluded(products::price)),
    ))
    .execute(&mut conn)?;

excluded comes from Diesel’s own diesel::upsert module. You can mix excluded(...) references with literals and column references in the SET clause, exactly like a normal update.

Pull the merged result straight back with .returning(...) — RETURNING works after ON CONFLICT:

let final_price: f64 = diesel::insert_into(products::table)
    .values((products::id.eq(1), products::name.eq("x"), products::price.eq(30.0)))
    .on_conflict(products::id)
    .do_update()
    .set(products::price.eq(excluded(products::price)))
    .returning(products::price)
    .get_result(&mut conn)?;

The conflict target (.on_conflict(products::id)) must correspond to a PRIMARY KEY or UNIQUE constraint in DuckDB, as with any ON CONFLICT statement.

See also

Insert

The base insert builder.

RETURNING

Read the merged row back.

Update

The SET-clause mechanics do_update reuses.

Transactions

Batch many upserts atomically.