Cross-field core keys — auto, from, on, domain

Four options on field / sub_field / conditional_field that read other parts of the input. Parsed once at compile time into __from_path__, __on_path__, __domain_ops__.

auto

Fill in a field from an MFA call.

field :id, :string, auto: {Ecto.UUID, :generate}
field :slug, :string, auto: {MyApp.Slug, :from_title, [:title]}

• In the default :add mode it runs whether or not the field was supplied, and replaces a value the caller sent. In :edit mode (builder({:root, attrs, :edit})) it runs only when the field is missing or nil, so a supplied value is kept.
• Compile-time verifier VerifyAutoMFA warns about an unknown MFA; the module still compiles.

from

Copy a value from a path elsewhere in the input.

field :user_id, :string, from: "headers::auth_user_id"
field :owner, :string, from: "root::username"

The string is split on :: into a path. A leading root starts at the whole input; any other path starts at the map the field sits in. The runtime reads get_in(input, [:headers, :auth_user_id]) and assigns it as the field's value.

• When the source holds a value, it replaces the field's own value, even one the caller sent.
• When the source is missing or nil, the field keeps its own value.

on

A pre-condition: the field is only allowed when another path is present. Useful for ownership ("role_id only present when role is sent").

field :role_id, :string, on: "role"
field :owner_id, :string, on: "root::owner"

The path follows the same rules as from. It is present when get_in/2 finds a value that is not nil. The value itself is not compared. The gate only runs when the field itself has a value; enforce: true makes the field required.

If the gate fails, the runtime emits a :dependent_keys error for the field.

To compare the value, use domain:

field :role_id, :string, domain: "!role=String[admin]"

domain

Cross-field constraint expressions. Compile-evaluated into a structured op map.

field :status, :string, domain: "!auth_type=Atom[admin, moderator]"

Reads: "when status has a value, auth_type must be :admin or :moderator". The ! prefix also makes a missing auth_type an error; ? lets it be missing. The values of Atom[...] / String[...] are separated by commas (enum= in a derives: string separates them with ::). Works in GuardedStruct.AshResource resources the same way as in standalone modules.

Pipeline order

Per do_pipeline/7 (lib/guarded_struct/runtime.ex):

1. normalize_keys — atom-convert keys (except dynamic_field values).
2. authorized_fields — reject unknown top-level keys when section opted in.
3. check_enforce_keys — required-fields check.
4. apply_auto — run the auto MFAs (:add replaces a supplied value, :edit keeps it).
5. check_domain — cross-field constraints.
6. check_on — conditional gates.
7. apply_from — copy from other paths.
8. Sub-field recursion.
9. Per-field validators.
10. run_main_validator.
11. Pass-1 derives over virtual fields.
12. Pass-2 derives over the merged map.