Draft for a post on Elixir School?

Ecto.Query.map_is_a_thing.md

%{ author: "Yuri Oliveira", author_link: "https://github.com/yuriploc", tags: , date: ~D[2021-11-12], title: "Writing a cleaner select query", excerpt: """ How learning more about the Ecto.Query module can help you write cleaner select queries. """ }

---

Writing a cleaner select query

It's not unusual for database queries to avoid select * when we can use an index (say, :id) and have a performance gain.

So, instead of writing:

query = from p in Post
Repo.all(query)

And getting back more data than we would care using, we can explicitly tell Ecto (and the DB) which columns we want it to return us:

query = from p in Post, select: %{id: p.id, title: p.title, category_id: p.category_id}
Repo.all(query)

But why do we have to be so explicit and duplicate keys and values? Isn't there a better way?

It turns out Ecto.Query already solved this for us with the [map/2](https://hexdocs.pm/ecto/Ecto.Query.API.html#map/2) function. So this:

query = from p in Post, select: %{id: p.id, title: p.title, category_id: p.category_id}
Repo.all(query)

Becomes:

query = from p in Post, select: map(p, [:id, :title, :category_id])
Repo.all(query)

Or, in Pipeland:

Post
|> select([p], %{id: p.id, title: p.title, category_id: p.category_id})
|> Repo.all()

Post
|> select([p], map(p, [:id, :title, :category_id]))
|> Repo.all()

And we can even have dynamic fields when using it in a function, like:

def filter_posts(post_ids, fields \\ [:id, :title, :category_id]) do
  Post
    |> select([p], map(p, ^fields))
    |> Repo.all()
end

Enjoy Ecto!

Thank you to the Groxio Mentoring folks for the support

This post looks GREAT! I'd suggest a catchier title, something that conveys that you'll be talking about the map Ecto query function. Maybe: "TIL: Cleaner Queries with Ecto map"

If you'd like to open this as a PR on Elixir School, you can follow these instructions to open your PR with the new blog post. https://github.com/elixirschool/elixirschool#posting-an-article. Feel free to @ me and I'll review it there!