Question

我只想用两个或多个列的简单连接来执行Ecto查询。

我认为以下elixir伪代码已经显示了我尝试做的事情：

customers = Customer.undeleted(
  from c in Customer,
  select: %{id: c.id, name: c.name <> " – " <> c.street},
  order_by: c.name
) |> Repo.all

它让我在SQL中疯狂，因为它很简单：...SELECT c.id, concat(c.name, ' - ', c,street) AS name

有关如何使用ecto querys解决此问题的任何想法？

Answer 1

您不能在Ecto中的选择表达式中使用<>。如果您想这样致电concat，可以使用fragment：

select: %{id: c.id, name: fragment("concat(?, ' - ', ?)", c.name, c.street)},

Answer 2

要添加到@Dogbert答案，您可以通过将SQL函数片段放在自定义宏中来清理代码，如docs中所述：

defmodule CustomSQLFunctions do
  defmacro concat(left, mid, right) do
    quote do
      fragment("concat(?, ?, ?)", unquote(left), unquote(mid), unquote(right))
    end
  end
end

然后导入以用于查询

import CustomSQLFunctions

customers = Customer.undeleted(
  from c in Customer,
  select: %{id: c.id, name: concat(c.name, ' - ', c.street)},
  order_by: c.name
) |> Repo.all

Elixir / Ecto / Postgres选择多个列作为一个列

2 个答案: