This is not so much a question but rather an answer to an issue which took me some time to figure out, and maybe a suggestion to improve the docs. Please see the end of the message for the solution.

Using Elixir 1.17.3, Ecto 3.12.4, Postgrex 0.19.3:

I’m working on some calendar-related functionality and needed to represent a duration. I had this in my migration:

create table(...) do
  ...
  add :default_duration, :interval
  ...

However, it was not clear to me how should I define my schema. For starters I was looking to make the field load as just a Postgrex.Interval, which seems to be the default and should not require any extra configuration.

I could not just do this:

schema "..." do
  ...  
  field :default_duration, :interval
  ...

as there is no :interval Ecto type. When I did this:

  field :default_duration, :duration

it compiled, but an attempt to load a record resulted in an error:

(ArgumentError) cannot load `%Postgrex.Interval{months: 0, days: 0, secs: 1800, microsecs: 0}` as type :duration for field :default_duration...

I tried field :default_duration, Postgrex.Interval, field :default_duration, Postgrex.Duration, even field :default_duration, Postgrex.Extensions.Interval with the same result.

Following the Ecto.Schema docs, Postgrex docs, Postgrex.Types.html#define/3 docs I tried to configure Postgrex to use the interval_decode_type: Duration, however the docs seemed rather sparse and it was not clear how exactly it should be done. I tried this in my lib/my_app/postgrex_types.ex:

Postgrex.Types.define(MyApp.PostgrexTypes, [], interval_decode_type: Duration)

However it is also additionally required passing types: MyApp.PostgrexTypes to Postgrex.start_link/1, and since I don’t start Postgrex manually, it was not clear to me how this bit could be passed to it in addition to all the existing Repo config.

It was not until I inadvertently looked into Ecto.Adapters.Postgres and saw the Extensions clause when it finally became clear. So, in order to employ the Duration Elixir type on a Postgres :interval field without registering any new extensions, the following worked:

In lib/my_app/postgres_types.ex:

Postgrex.Types.define(MyApp.PostgresTypes, Ecto.Adapters.Postgres.extensions(), interval_decode_type: Duration)

In config/config.exs:

config :my_app, MyApp.Repo,
  ...
  types: MyApp.PostgresTypes

At this point a schema field defined as

  field :default_duration, :duration

started returning the desired Duration struct:

  ...
  default_duration: %Duration{minute: 30, microsecond: {0, 6}},
  ...

Maybe the solution is so obvious no one really had an issue before - I didn’t find any complaints online. But maybe the Ecto.Schema | Primitive types should be updated to include a more explicit example.