JoinRelationOptions:
    | boolean
    | Record<string, never>
    | { _apply?: (builder: JoinApplyBuilder) => JoinApplyBuilder }

Options for a single relation in a join specification.

// Simple join (load all related records)
{ files: true }
// Complex join with filter
{ files: { _apply: (qb) => qb.filterWhere({ completed: true }) } }