I have a Recipe model which has_many Ingredients (which in turn belongs_to Recipe). I want Ingredient to be existent dependent on Recipe; an Ingredient should never exist without a Recipe.

I'm trying to enforce the presence of a valid Recipe ID in the Ingredient. I've been doing this with a validates :recipe, :presence => true (Rails 3) statement in Ingredient. This works fine if I save the Recipe before adding an Ingredient to it's ingredients collection. However, if I don't have explicit control over the saving (such as when I'm creating a Recipe and its Ingredients from a nested form) then I get an error:

Ingredients recipe can't be blank

I can get around this simply by dropping the presence validation on Ingredient.recipe. However, I don't particularly like this, as it means I'm working without a safety net.

What is the best way to enforce existence-dependence in Rails?

Things I'm considering (please comment on the wisdom of each):

• Adding a not-null constraint on the ingredients.recipe_id database column, and letting the database do the checking for me.
• A custom validation that somehow checks whether the Ingredient is in an unsaved recipe's ingredient collection (and thus can't have a recipe_id but is still considered valid).