Most efficient way to replace some values of a struct by other values of that struct

Questions / Help
1

I have a list of structs as a result of a Repo.all() and I would like to copy some of their values to other key values.
I made up this code to illustrate what I intend:

fields_base = [:date, :text, :title]            #list with the keys I want to update their values
fields_langs = [:date_pt, :text_pt, :title_pt]  #list with the keys that contain the values I want to copy
        events = Data.listAll(Events)
        events = 
          for i <- events do
            %{i | i.fields_base[0] => i.fields_langs[0] }
          end
        events =
          for i <- events do
            %{i | i.fields_base[1] => i.fields_langs[1] }
          end
        events = 
          for i <- events do
            %{i | i.fields_base[2] => i.fields_langs[2] }
          end

I’m sure that Map could be useful here but I don’t know how to use it.

2

Instead of two lists with the keys, it’s better to use a map in order to make the relationship direct, and easier to enumerate over:

fields = %{date: :date_pt, text: :text_pt, title: :title_pt}

You only need to iterate over the events once. There are a number of ways to write this. Here’s one:

for i <- events do                                                                      
  fields
  |> Enum.map(fn {k,v} -> {k, Map.get(i, v)} end)
  |> Enum.into(Map.from_struct(i))
end

However, since your are using Ecto, you could also handle the renaming directly in the query.

3

Great!
Yes, as I’m learning I’m taking baby steps in here to get solid learning of the future but I realise I have two way of doing this:
a) with the query results, pure Elixir, no Ecto - what you just showed;
b) in the query itself - maybe more efficient because it’s just one step instead of two.
I’m still struggling with b). Do you have an alternative to a simple Repo.all(Events)?

4

There are some examples in the documentation for Ecto.Query.select/3.

Something similar to this:

import Ecto.Query
query = from(e in Event, select: %{title: e.title_pt, date: e.date_pt, text: e.text_pt})
events = Repo.all(query)

Note that this will only select those three fields for each Event record. You also end up with a map for each record, not an Event struct.