nullify on a not-null column should set to default value

[jrmhaig]

If a field is set as follows:

amoeba do
  enable
  nullify :test_field
end

and test_field is set as not null in the database then the value of a duplicate should be the default value, not null. However, at present, the field is set to nil and the resulting instance cannot be saved.

To reproduce

Create a migration:

class CreateTestModels < ActiveRecord::Migration[6.1]
  def change
    create_table :test_models do |t|
      t.string :test_string, null: false, default: 'Test default'

      t.timestamps
    end
  end
end

and the corresponding model:

class TestModel < ApplicationRecord
  amoeba do
    enable
    nullify :test_string
  end
end

Then:

irb> test = TestModel.create
irb> test.test_string
=> "Test default"
irb> dup = test.amoeba_dup
irb> dup.save

Expected behaviour: dup is saved successfully with test_string set to "Test default".
Actual behaviour: Not-null violation exception raised.

[nateleavitt]

Also experiencing this bug. I have a jsonb column with a default value of an empty Hash. Upon duplicating, the field is nil instead of {}.