An error occurred while saving entities that do not expose foreign key properties for their relationships

For those of you who would still have this error with all keys properly defined, have a look at your entities and make sure you don't leave a datetime field with a null value.

This error message can be thrown for any kind of reason. The 'InnerException' property (or its InnerException, or the InnerException of that, etc) contains the actual primary cause of the problem.

It would of course be useful to know something about where the problem occurred - which object(s) in the unit of work is causing the problem? The exception message would normally tell you in the 'EntityEntries' property, but in this case, for some reason, that can't be done. This diagnostic complication - of the 'EntityEntries' property being empty - is apparently because some Entities 'do not expose foreign key properties for their relationships.'

Even if the OP gets the error because of failing to initialize DateTimes for the second instance of User, they get the diagnostic complication - 'EntityEntries' being empty, and a confusing top-level message ... because one of their Entity's doesn't 'expose foreign key properties'. To fix this, Avatar should have a public virtual ICollection<User> Users { get; set; } property defined.

The issue was resolved by adding an FK property.