query: aliases are not recognized within `splice/1`

[Gladear]

Elixir version

Elixir 1.15.6 (compiled with Erlang/OTP 26)

Database and Version

N/A

Ecto Versions

3.11.1

Database Adapter and Versions (postgrex, myxql, etc)

N/A

Current behavior

As far as I see, it's not possible to use "aliases" within splice/1

iex> Mix.install(ecto: "3.11.1")
iex> import Ecto.Query
iex> from seq in "pg_class",
...>     where: seq.relkind == "S",
...>     select: %{
...>       seq_name: fragment("concat_ws(?, ?)", ".", splice(^["public", seq.relname]))
...>     }
error: undefined variable "seq"
  iex:6

** (CompileError) cannot compile code (errors have been logged)

outputs

error: undefined variable "seq"
  iex:6

** (CompileError) cannot compile code (errors have been logged)

Obviously, this is a minimal reproduction. My goal would be to define a macro for concat_ws

  defmacro concat_ws(separator, expressions) do
    quote bind_quoted: [separator: separator, expressions: expressions] do
      fragment("concat_ws(?, ?)", separator, splice(^expressions))
    end
  end

Then use it like this

select: %{
  seq_name: concat_ws(".", ["public", seq.relname])
}

Expected behavior

I expect aliases to be recognized within splice/1

[josevalim]

I don't think this can work indeed. We should probably document that we can only splice runtime values.

[greg-rychlewski]

If there are no conflicts in column names you could potentially do this

seq_name: fragment("concat_ws(?, ?)", ".", splice(^["public", "relname"]))

[LostKobrakai]

Could aliases be made to work by supporting dynamic within the spliced list? My usecase is sqlite's json_object() function, which needs the parameters to alternate between key value and the corresponding value for the object key.