iex.log

iex(2)> import Ecto.Query
Ecto.Query
iex(3)> Video |> select([v], v.id)
#Ecto.Query<from v0 in DB.Schema.Video, select: v0.id>
iex(4)> Video |> select([v], v.id) |> DB.Repo.all()
[debug] QUERY OK source="videos" db=1.9ms decode=2.6ms queue=6.5ms
SELECT v0."id" FROM "videos" AS v0 []
[1]
iex(5)> Video |> select([v], v.id) |> DB.Repo.all() |> Enum.map(&Video.changeset_generate_hash_id/1)     
[debug] QUERY OK source="videos" db=0.3ms
SELECT v0."id" FROM "videos" AS v0 []
** (FunctionClauseError) no function clause matching in DB.Schema.Video.changeset_generate_hash_id/1    
    
    The following arguments were given to DB.Schema.Video.changeset_generate_hash_id/1:
    
        # 1
        1
    
    Attempted function clauses (showing 1 out of 1):
    
        def changeset_generate_hash_id(video = %{id: id})
    
    (db) lib/db_schema/video.ex:157: DB.Schema.Video.changeset_generate_hash_id/1
    (elixir) lib/enum.ex:1327: Enum."-map/2-lists^map/1-0-"/2
iex(5)> DB.Schema.Video.changeset_generate_hash_id(%DB.Schema.Video{id: 42, hash_id: nil})
#Ecto.Changeset<
  action: nil,
  changes: %{hash_id: "GMAA"},
  errors: [],
  data: #DB.Schema.Video<>,
  valid?: true
>
iex(6)> 

video.ex

  def changeset_generate_hash_id(video = %{id: id}) do
    change(video, hash_id: VideoHashId.encode(id))
  end
  

Video |> select([v], map(v, [:id])) |> DB.Repo.all()

[debug] QUERY OK source="videos" db=2.0ms queue=1.8ms
SELECT v0."id" FROM "videos" AS v0 []
[%{id: 1}]