MartenStorages::Attachable

Mixin for host target models — the records that own attachments (Book/Picture/Markdown in Writebook). Adds query sugar for fetching attachments on the calling record. The actual attach/save logic lives in Service.

Usage:

class Book < Marten::Model include MartenStorages::Attachable

# ...other fields...

# Tell the mixin which concrete Attachment model class to query.
attachment_model ::DocumentAttachment

end

Adds:

• book.attachments_for("cover") — Array(Attachment) for all originals of this name (ordered by created_at asc).
• book.attachment_for("cover") — most-recent original (or nil).
• book.variants_of(attachment, "thumbnail") — variant row for the given original + variation_kind (or nil).

attachment_model is a class-level keyword that captures the host's concrete polymorphic Attachment class. Marten's polymorphic to: list is compile-time fixed, so the host app owns the concrete Attachment model with its own to: [...] list — the shard just threads queries through whatever class the host names.

The generated methods delegate to MartenStorages::Service so the query shape lives in one place (review §10 / §11) — extending the filter chain (e.g. soft-delete) happens in Service and the mixin picks it up for free.