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Resolves all 4 parts of Issue #3 
Resolves part 2 of Issue #10 
Resolves all 3 parts of Issue #11 
You can validate the user and the review directly rather than the IDs:

```ruby
validates :user, presence: true
validates :review, presence: true
```

This ensures that it not only contains a value, but that the ID refers to an actual user and review object.