ash/documentation/topics/identities.md

Used for indicating that some set of attributes uniquely identify a resource.

This will allow these fields to be passed to `c:Ash.Api.get/3`, e.g `get(Resource, [some_field: 10])`,
if all of the keys are filterable. Otherwise they are purely descriptive at the moment.
The primary key of the resource does not need to be listed as an identity.

eager check with

  The identity is checked on each validation of the changeset. For example, if you are using
  `AshPhoenix.Form`, this looks for a conflicting record on each call to `Form.validate/2`.
  For updates, it is only checked if one of the involved fields is being changed.

  For creates, The identity is checked unless your are performing an `upsert`, and the
  `upsert_identity` is this identity. Keep in mind that for this to work properly, you will need
  to pass the `upsert?: true, upsert_identity: :identity_name` *when creating the changeset* instead of
  passing it to the Api when creating.

  The `primary?` action is used to search for a record. This will error if you have not
  configured one.

pre check with

  Behaves the same as `eager_check_with`, but it runs just prior to the action being committed. Useful for
  data layers that don't support transactions/unique constraints, or manual resources with identities.