I'm trying to add the sum of all ratings to a post. But I'm getting an error: `ArithmeticError: bad argument in arithmetic expression`. I believe that's because not all `Post` will have `ratings` associated with it. And then it won't be able to `sum` the values in those non-existent ratings. 

How would I go about setting the rating to `0` if there are no `ratings` associated with a `post` in the database?

Query:

```
from post in Post,
left_join: rating in assoc(post, :ratings),
group_by: post.id,
select: %{post | rating: sum(rating.value)}

```

Schemas:

```
schema "posts" do
    field :rating, :integer, virtual: true
    has_many :ratings, MyApp.Rating
end

schema "ratings" do
    field :value, :integer
    belongs_to :post, MyApp.Post
end

```