[Plone-conference] Porting Products to Plone 5: reference from custom AT Type to Document breaks

[Kees Hink]

Products.Archetypes' ReferenceEngine.py has a _uidFor() method that, 
when passed a string, tries to get the UID from the uid_catalog. [1]

Problem: In Plone 5, when creating a reference from a custom content 
type object to a Document, the target UID isn't in uid_catalog. It's in 
portal_catalog. The custom types' UIDs are in uid_catalog. This surfaced 
while trying out Products.Doormat on Plone 5 [2]

Links between default Plone content type objects work fine.

What did i miss?

[1] 
https://github.com/plone/Products.Archetypes/blob/master/Products/Archetypes/ReferenceEngine.py#L527

[2] https://github.com/collective/Products.Doormat/tree/plone5

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