Today, I suffered the same problem as you met.
This is what I figured out.
'containers' function add '__parent__' attribute to IMyContent
interface, like you did to IMyContentContained interface.
It means that new implementation of IMyContent has '__parent__',
while old implementation didn't. the difference causes the error.
Now, addform will try to render '__parent__', because IMyContent
has it. however, the view of '__parent__' is never found. then
the error raised.
If you want to omit '__parent__', you have to specify that
using 'fields' attribute of 'addform' directive in your zcml,
or old implementation might be better.
I think that this situation lessen the value of
'contains'/'containers' functions. I wonder if I understand
things correctly. it might be some other way.
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