Ecto Migration: `not_null_violation` when modifying column null->non-null & adding default value

Questions & Help Questions
,
1

Howdy y’all,

I’m brand new to Elixir and have what is likely a simple problem. I created an Ecto schema with a votes column that is type :integer and nullable. I later realized it should be non-nullable and default to 0. I made a migration to perform this update, however I am getting the error:

** (Postgrex.Error) ERROR 23502 (not_null_violation) column "votes" of relation "faqs" contains null values

Migration

defmodule Pento.Repo.Migrations.AlterFaqsAddDefaultVoteValue do
  use Ecto.Migration

  def change do
    alter table(:faqs) do
      modify :votes, :integer, default: 0, null: false
    end
  end
end

See below for my existing table and full error output. I found a comment from Jose Valim stating “The default value in migration is used to set the value for existing rows.” in 2015. I read that as stating that the default value in a migration should change any existing null fields to the stipulated value. Is that not the case and should I just use a separate execute for altering existing rows?

Existing Table

id question answer votes inserted_at updated_at
1 Question 1 Answer 1 NULL 2024-02-04 02:54:39 2024-02-04 12:51:01
2 Question 2 Answer 2 NULL 2024-02-04 02:56:24 2024-02-04 02:56:24 
CREATE TABLE "public"."faqs" (
    "id" int8 NOT NULL DEFAULT nextval('faqs_id_seq'::regclass),
    "question" varchar(255),
    "answer" varchar(255),
    "votes" int4,
    "inserted_at" timestamp(0) NOT NULL,
    "updated_at" timestamp(0) NOT NULL,
    PRIMARY KEY ("id")
);

Output

07:05:36.729 [info] == Running 20240204125254 Pento.Repo.Migrations.AlterFaqsAddDefaultVoteValue.change/0 forward

07:05:36.730 [info] alter table faqs
** (Postgrex.Error) ERROR 23502 (not_null_violation) column "votes" of relation "faqs" contains null values

    table: faqs
    column: votes
    (ecto_sql 3.11.1) lib/ecto/adapters/sql.ex:1054: Ecto.Adapters.SQL.raise_sql_call_error/1
    (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
    (ecto_sql 3.11.1) lib/ecto/adapters/sql.ex:1161: Ecto.Adapters.SQL.execute_ddl/4
    (ecto_sql 3.11.1) lib/ecto/migration/runner.ex:348: Ecto.Migration.Runner.log_and_execute_ddl/3
    (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
    (ecto_sql 3.11.1) lib/ecto/migration/runner.ex:311: Ecto.Migration.Runner.perform_operation/3
    (stdlib 5.2) timer.erl:270: :timer.tc/2
    (ecto_sql 3.11.1) lib/ecto/migration/runner.ex:25: Ecto.Migration.Runner.run/8

Thank you for any help!