Attempt1.rb

def change
  create_table :ingredients do |t|
    t.integer :quantity, default: 0, null: false
    t.timestamps
  end
end

Attempt2.rb

def change
  create_table :ingredients do |t|
    t.integer :quantity, default: 1, null: false
    t.timestamps
  end
end

Attempt3.rb

add_column :ingredients, :quantity, :integer, :default => 1, :null => false

Attempt4.rb

change_column_null :ingredients, :quantity, false, 1

Testing.rb

Loading development environment (Rails 3.2.11)
>> ingredient = Ingredient.new
=> #<Ingredient id: nil, product_id: nil, quantity: nil, unit_id: nil, recipe_id: nil, created_at: nil, updated_at: nil>
>> ingredient.quantity
=> nil
>> ingredient.save
   (0.2ms)  BEGIN
  SQL (10.9ms)  INSERT INTO "ingredients" ("created_at", "product_id", "quantity", "recipe_id", "unit_id", "updated_at") VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"  [["created_at", Thu, 31 Jan 2013 09:35:25 CET +01:00], ["product_id", nil], ["quantity", nil], ["recipe_id", nil], ["unit_id", nil], ["updated_at", Thu, 31 Jan 2013 09:35:25 CET +01:00]]
   (1.7ms)  COMMIT
=> true
>> ingredient = Ingredient.last
  Ingredient Load (1.0ms)  SELECT "ingredients".* FROM "ingredients" ORDER BY "ingredients"."id" DESC LIMIT 1
=> #<Ingredient id: 1, product_id: nil, quantity: nil, unit_id: nil, recipe_id: nil, created_at: "2013-01-31 08:35:25", updated_at: "2013-01-31 08:35:25">
>> 

I'm an idiot. rake db:reset does not run migrations, just loads schema.rb.