Steps to Reproduce:
Create a new model, the type NHibernate;
Add a class with any property ('ll call Class1);
Add another class (Class2) inheriting from Class1;
Place in Class1 and Class2 the discriminator;
Add to Class2 Join one with some table;
Place in the propridades Class2 Join the created anteriorly;
Try an Update Model From Database;
Occurs the error: "The given key was not present in the dictionary."
Version: 5.7.276
Is modeled with the problem:
https://mega.co.nz/#!FURyFZ7Q!sXcGiBn2X ... wzBaZyDdAo
The sample was also sent by email.