How to write a migration to convert an already-present association to optional?

Question

I have a few belongs_to migrations created&migrated but I see now that in Rails 5, the default behavior is required: true which I do not want.<

Answer 1

You can use the change_column_null method for that:

def change
  change_column_null :table, :column, true
end

change_column_null is reversible, if you need to rollback the value will be false (or true depending on the case).

---

Answering to your question If I simply add optional: true in the model, will it work?. No, if you have a foreign key with nullable value NOT NULL you're going to face an error everytime you try to create a record without that value, e.g:

A table comments with a relationship to users:

                                                              Table "public.comments"
   Column    |              Type              | Collation | Nullable |               Default                | Storage  | Stats target | Description
-------------+--------------------------------+-----------+----------+--------------------------------------+----------+--------------+-------------
 id          | bigint                         |           | not null | nextval('comments_id_seq'::regclass) | plain    |              |
 description | character varying              |           |          |                                      | extended |              |
 user_id     | bigint                         |           | not null |                                      | plain    |              |
 created_at  | timestamp(6) without time zone |           | not null |                                      | plain    |              |
 updated_at  | timestamp(6) without time zone |           | not null |                                      | plain    |              |
Indexes:
    "comments_pkey" PRIMARY KEY, btree (id)
    "index_comments_on_user_id" btree (user_id)
Foreign-key constraints:
    "fk_rails_03de2dc08c" FOREIGN KEY (user_id) REFERENCES users(id)

When you instantiate a new record, Rails will tell nothing about the missing foreign key:

foo = Comment.new(description: :foo)
foo.valid? # true
foo.save
# ActiveRecord::NotNullViolation: PG::NotNullViolation: ERROR:  null value in column "user_id" violates not-null constraint

But if you try to persist the record, you're going to get an ActiveRecord::NotNullViolation error because of the not-null constraint.