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Only get specific fields when retrieving data with Ecto in Phoenix

I am trying to return some JSON data to one of my API calls in Phoenix. I retrieve all the Subject entries and submit them, but Ecto returns some additional fields that I don't need.

What can I do for:

  • Get only certain attributes (e.g. id and name )
  • Do not get unnecessary fields in my answer (e.g. __meta__ and __owner__ )

This is my Controller :

 # Controller def index(conn, _) do subjects = Subject |> Repo.all conn |> render subjects: subjects end

This is my View :

 # View def render("index.json", %{subjects: subjects}) do subjects end

This is my answer:

 [ { "teachers": { "__owner__": "Elixir.MyApp.Subject", "__field__": "teachers", "__cardinality__": "many" }, "updated_at": "2015-06-20T15:32:20Z", "topics": { "__owner__": "Elixir.MyApp.Subject", "__field__": "topics", "__cardinality__": "many" }, "name": "Physics", "inserted_at": "2015-06-20T15:32:20Z", "id": 1, "__meta__": { "state": "loaded", "source": "subjects" } }, { "teachers": { "__owner__": "Elixir.MyApp.Subject", "__field__": "teachers", "__cardinality__": "many" }, "updated_at": "2015-06-20T15:37:59Z", "topics": { "__owner__": "Elixir.MyApp.Subject", "__field__": "topics", "__cardinality__": "many" }, "name": "Chemistry", "inserted_at": "2015-06-20T15:37:59Z", "id": 2, "__meta__": { "state": "loaded", "source": "subjects" } }, { "teachers": { "__owner__": "Elixir.MyApp.Subject", "__field__": "teachers", "__cardinality__": "many" }, "updated_at": "2015-06-20T15:38:41Z", "topics": { "__owner__": "Elixir.MyApp.Subject", "__field__": "topics", "__cardinality__": "many" }, "name": "Mathematics", "inserted_at": "2015-06-20T15:38:41Z", "id": 3, "__meta__": { "state": "loaded", "source": "subjects" } }, { "teachers": { "__owner__": "Elixir.MyApp.Subject", "__field__": "teachers", "__cardinality__": "many" }, "updated_at": "2015-06-22T15:40:17Z", "topics": { "__owner__": "Elixir.MyApp.Subject", "__field__": "topics", "__cardinality__": "many" }, "name": "Biology", "inserted_at": "2015-06-22T15:40:17Z", "id": 4, "__meta__": { "state": "loaded", "source": "subjects" } } ]
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1 answer

Change your view to:

 def render("index.json", %{subjects: subjects}) do Enum.map(subjects, &Map.take(&1, [:id, :name])) end

Alternatively, you can also ask Ecto to return a subset of the fields by changing your controller to:

 def index(conn, _) do subjects = from(s in Subject, select: %{id: s.id, name: s.name}) |> Repo.all conn |> render subjects: subjects end
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