# `Diffo.Provider.Extension.Verifiers.VerifyProviderDomain`
[🔗](https://github.com/diffo-dev/diffo/blob/v0.9.0/lib/diffo/provider/extension/verifiers/verify_provider_domain.ex#L5)

Verifies that a provider resource (Instance / Party / Place — anything composing
`Diffo.Provider.Extension`) carries the `:Provider` Neo4j label.

`:Provider` is what makes provider polymorphism work: cross-world projection
(`Diffo.Provider.get_place_by_id!/1` and friends, `AshNeo4j.worlds/1`) and `PlaceRef` / `PartyRef` /
`belongs_to` resolution all MATCH on `[:Provider, <base-type>]`. A node gets `:Provider`
either because its domain *is* `Diffo.Provider` (the built-in leaves — `:Provider` is
their `domain_label`) or because its domain composes `Diffo.Provider.DomainFragment`
(which writes `:Provider` as the domain-fragment label).

Forget the fragment on a consumer domain and the node simply lacks `:Provider`: provider
readers stop finding it and projection **silently** returns nothing. This verifier turns
that silent footgun into a compile-time error pointing at the fix.

## Resolution (and why it isn't the persisted `:all_labels`)

We can't trust the `:all_labels` AshNeo4j persists, because its domain-fragment slot is
resolved with `Code.ensure_loaded?` at compile time — if the domain hasn't loaded yet
(resource/domain compile order), it bakes a `nil` fragment label even though the node
*does* get `:Provider` at runtime (where `domain_fragment_label/1` falls back to the
domain). So we resolve the domain ourselves:

  * `domain_label == :Provider` → fine (built-in / `Diffo.Provider` domain).
  * otherwise resolve the domain (compiling it if needed) and check it emits `:Provider`
    via `AshNeo4j.DataLayer.Domain.Info.neo4j_label/1`.

If the domain genuinely can't be resolved at this point in compilation, we **stay silent**
— a best-effort guard never false-positives on a resource that may be correct at runtime.
In practice the domain is available by the time a consumer's resource compiles, so the
forgotten-fragment case is caught.

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