# `MishkaCmsCore.Runtime.Resources.Validations.AnchorSiteRequired`
[🔗](https://github.com/mishka-group/mishka_cms_core/blob/v0.0.1/lib/mishka_cms_core/runtime/resources/__validations/anchor_site_required.ex#L5)

A page and an error page are never global; a layout and a component can be.

`MishkaCmsCore.Runtime.PageBuilderEvent` anchors to all four through one polymorphic
`source_type`, so it is `global? true` — a component's history has to be writable with a `nil`
`site_id`. That makes the resource unable to say, by its multitenancy alone, the thing that is
nonetheless true of half its rows:

    source_type in [:page, :error_page]  ⟹  site_id is present

Verified against the resources themselves rather than assumed: `Page` and `ErrorPage` declare
`global? false`, `Layout` and `Component` declare `global? true`.

Paired with a check constraint of the same name on the table. This catches the bug with a message;
the constraint catches the code path that skipped the validation — worth having both, because these
rows are written from the editor's hot path by batched, retried upserts, which is exactly the sort
of path that grows a shortcut.

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