def message
<<~MSG
#{@sideload.parent_resource}: Tried to find a Resource with type '#{@name.inspect}', but did not find one!
This is because either a Resource with that type doesn't exist, or it's not registered on the sideload. The below example shows how to register a Resource with this sideload. Make sure one of the registered Resources has type '#{@name.inspect}'
polymorphic_belongs_to #{@sideload.name.inspect} do
group_by(#{@sideload.grouper.field_name.inspect}) do
on(:foo)
on(:foo).belongs_to :foo, resource: FooResource # (long-hand example)
end
end
MSG
end