Elixir，Ecto模式匹配条件与db查询的行为不符合预期 [英] Elixir, Ecto pattern matching conditional with db query not behaving as expected

问题描述

如果该记录不存在，我希望此条件可以创建它，但是它不....返回nil。

If the record does not exist, I would expect this conditional to create it, but it does not.... nil is returned.

case Repo.get_by(User, %{email: "[e-mail]"}) do
    struct ->
      struct
    nil ->
      params = Map.merge(%{email: "[e-mail]"}, %{password: "password"})
      Repo.insert!(User.changeset(User.__struct__, params))
end

# returns nil.... huwutt???

但是，如果我更改条件的顺序，它就可以工作。我在这里想念什么？

However, if I change the ordering of the condition, it works. What am I missing here?

case Repo.get_by(User, %{email: "[e-mail]"}) do
    nil ->
      params = Map.merge(%{email: "[e-mail]"}, %{password: "password"})
      Repo.insert!(User.changeset(User.__struct__, params))
    struct ->
      struct
end

# returns a set of 24" pythons, brother.... huzah!

推荐答案

根据文档

案例允许我们将值与许多模式进行比较，直到我们
找到匹配的模式为止：

case allows us to compare a value against many patterns until we find a matching one:

在另一个模式中单词，第一个匹配的案例将运行，而 case 不会继续进行。

In another word, the first matching case will run and case will not proceed further.

在您的第一个示例中，第一个因为您不提供任何保护，所以大小写总是匹配的，因此 struct 将绑定到 nil 。第二种方法解决了问题，因为您首先要对特定情况进行模式匹配，然后通过将 case 的求值绑定到 struct来默认为一般情况。

In your first example, the first case will always be matched since you are not providing any guards, and as such, struct will bind to nil. Your second approach solves the problem because you're pattern matching a specific case first, and then defaulting to a general case by binding the evaluation of case to struct.

还请注意，您可以使用gua rds在您的第一种情况下，请确保 struct 的值是 map 的此处。

Also note that you can use guards in your first case to make sure that the value of struct is a map as outlined here.

case Repo.get_by(User, %{email: "[e-mail]"}) do
    struct when is_map(struct) ->
      struct
    nil ->
      params = Map.merge(%{email: "[e-mail]"}, %{password: "password"})
      Repo.insert!(User.changeset(User.__struct__, params))
end

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