在关联列上使用 sum 时的 ArithmeticError

Question

我正在尝试将所有评分的总和添加到 post。但我收到一个错误：ArithmeticError: bad argument in arithmetic expression。我相信那是因为并非所有 Post 都会有 ratings 与之关联。然后它将无法 sum 那些不存在的评级中的值。

如果数据库中没有与 post 关联的 ratings，我该如何将评分设置为 0？

查询：

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

架构：

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

Answer 1

感谢 Elixir Slack 频道，我设法使用 postgres 函数解决了这个问题 coalesce()。

The COALESCE function returns the first of its arguments that is not null. Null is returned only if all arguments are null. It is often used to substitute a default value for null values when data is retrieved for display.

有了这些新知识，我重写了我的查询，它似乎按预期工作：

from post in Post,
left_join: rating in assoc(post, :ratings),
group_by: post.id,
select: %{post | rating: sum(fragment("coalesce(?,0)", rating.value))}

Answer 2

Ecto

中还有一个coalesce功能

from post in Post,
left_join: rating in assoc(post, :ratings),
group_by: post.id,
select: %{post | rating: sum(coalesce(rating.value, 0))}

在关联列上使用 sum 时的 ArithmeticError

ArithmeticError when using sum on an associated column

ecto