bulkCreate returning option dialect limitations

Direct answer

In Sequelize 6.x, bulkCreate(rows, { returning: true }) is dependable primarily for PostgreSQL, where Sequelize maps it to INSERT ... RETURNING and hydrates instances with server-generated values. For MySQL, MariaDB, and SQLite, there is no standardized guarantee that returning: true returns full created rows. The resolved array typically contains the instances you passed in, possibly with auto-increment IDs assigned back by Sequelize, but database defaults, timestamps, and trigger-generated values are often missing. The exact shape depends on the installed Sequelize patch release and dialect driver.

Confirmed facts versus likely explanation

• Confirmed: PostgreSQL supports row-level returning through RETURNING; MySQL, MariaDB, and SQLite do not expose the same bulk-insert returning semantics uniformly across Sequelize 6.x.
• Confirmed: Sequelize 6.x does not offer a single option that forces identical full-row results from bulkCreate across all supported dialects.
• Likely explanation: When the dialect lacks native returning, Sequelize cannot populate instances from the insert result itself, so it returns input instances with only the identifiers it can assign locally.

Steps for this case

1. Run bulkCreate with returning: true on your target dialect and inspect the resolved instances with instance.get({ plain: true }) to see which columns are populated.
2. If you need server-generated values portably, add a batch identifier column or capture returned IDs, then run a scoped SELECT for those IDs after insertion.
3. On MySQL 8.0.19+, native INSERT ... RETURNING exists, but do not assume Sequelize 6.x uses it automatically for bulkCreate; verify the generated SQL for your driver version.

Verification

const created = await Model.bulkCreate(rows, { returning: true });
console.log(created.map((m) => m.get({ plain: true })));
// Compare against: await Model.findAll({ where: { id: created.map((m) => m.id) } });

Treat this as a draft pending verification against your exact dependency versions. Missing diagnostic detail that changes the recommendation: which exact Sequelize version and MySQL/MariaDB/SQLite driver are you using, and do you need server-generated defaults or only auto-increment IDs?