How to change column to nullable with modify in Ecto migration

Sooner or later you'll have to change the null constraint in one of your DB relations. How to do it easily in Ecto?

Although I came across many different examples where a raw SQL has been used to perform this type of operation in Ecto, it's actually super easy to do it with modify/3 function.

Let's assume that your migration looks like this:

create table(:blog_posts) do
  add :title, :string, null: false
  add :intro, :text, null: false
  add :body, :text, null: false
  add :category_id, references(:blog_categories, on_delete: :delete_all)
end

... and at some point, you realize that you don't want to force passing the intro column value.

You can change it easily this way:

alter table(:blog_posts) do
  modify :intro, :text, null: true, from: :text
end

It's also worth mentioning that it's possible to modify foreign keys with modify:

alter table(:blog_posts) do
  modify :category_id,
    references(:blog_categories, on_delete: :delete_all),
    null: false,
    from: references(:blog_categories, on_delete: :delete_all)
end