cross_ref_source_key_unknown

The :source-key declared on a provider-route (cross-ref …) is not a string-typed key on the target form.

A provider-route cross-ref reads the string under :source-key from each :target instance and hands it to the provider. This fires when the target form declares no such key, or declares it with a type that is not string-shaped. Either way no instance could ever supply the provider with anything, so the member set would silently be empty.

:source-key defaults to src; declare the key on the target form, or point :source-key at the key that actually holds the content.

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Run sjon explain cross_ref_source_key_unknown to read this at a terminal, browse every code, or work through diagnostics-driven repair if you want the habit rather than the answer.