What you are trying to achieve is impossible with current version of Entity Framework. Quoting from https://stackoverflow.com/a/7022799/337294:
It is not possible. Relations in EF follows exactly same rules as in the database. It means that principal table must have unique identifier which is referenced by dependent table. In case of database the identifier can be either primary key or unique column(s) of principal table. Otherwise it is not valid relation.
And since Entity Framework does not support unique indexes yet (despite strong demand in their Feature Suggestion page), it has to be the Id
property of your Table1
class.