# `Ash.Resource.Change.AtomicSet`
[🔗](https://github.com/ash-project/ash/blob/v3.27.8/lib/ash/resource/change/atomic_set.ex#L5)

A change that sets an attribute using an expression evaluated in the data layer during create.

This is used by the `atomic_set/3` builtin change. Unlike the change used by
`atomic_update/3`, this is for setting values during the INSERT phase of create
actions, not for updating existing values.

The expression cannot reference existing database values via `atomic_ref/1` since there
is no existing row during creates.

When used on update actions, behaves the same as `atomic_update/3`.

## Return Value

The `atomic/3` callback returns `{:atomic_set, atomics}` which tells Ash to apply
these values during the INSERT phase (stored in `changeset.create_atomics`).

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*Consult [api-reference.md](api-reference.md) for complete listing*