Elixir / Ecto / Postgres选择多个列作为一个列

Question

I just want do execute an Ecto query with an simple concatination of two or more columns.

I think the following elixir pseudo code already shows what I try to do:

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

It drives me crazy cuz in SQL it is easy like this: ...SELECT c.id, concat(c.name, ' - ', c,street) AS name

Any ideas how to solve this with ecto querys ?

Answer 1

You cannot use <> in a select expression in Ecto. If you want to call concat like that, you can use fragment :

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

Answer 2

To add to @Dogbert answer, you can clean up the code a bit by putting the SQL function fragment in a custom macro as described in the docs :

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

Then import for use in query

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