Field required even though null: true - ruby-on-rails

Question

I'm trying to add some seed data to my database, but ActiveRecord won't let me create a category without an associated user.
ActiveRecord::Schema.define(version: 2020_07_18_233635) do
# These are extensions that must be enabled in order to support this database
enable_extension "plpgsql"
create_table "categories", force: :cascade do |t|
t.string "name"
t.bigint "user_id"
t.string "image_url"
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
t.index ["user_id"], name: "index_categories_on_user_id"
end
create_table "users", force: :cascade do |t|
t.string "email", default: "", null: false
t.string "encrypted_password", default: "", null: false
t.string "reset_password_token"
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
t.index ["email"], name: "index_users_on_email", unique: true
t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
end
add_foreign_key "categories", "users"
add_foreign_key "goals", "categories"
end
When I try to create a category:
category = Category.create!({name: 'Fitness', image_url: 'some_url'})
I get this error:
ActiveRecord::RecordInvalid (Validation failed: User must exist)
EDIT:
Here's my Category class:
class Category < ApplicationRecord
belongs_to :user
has_many :goals
end

Answer 1

Prior to Rails 5, the default for a belongs_to association was that it was optional. However, for Rails >= 5, the reverse is true; the presence of an associated object is checked by default. Specify :optional on the association to opt out of that:
class Category < ApplicationRecord
belongs_to :user, optional: true
has_many :goals
end