select single association from has_many through in Rails

Question

Is there any (clean) way to access a single association when using a has_many through association?

For example, to get all of a Recipe's Ingredients, I can write:

@ingredients = @recipe.ingredients

However, if I have an ingredient already selected, and I want to add data to the join table for the recipe/ingredient, I can't figure out the clean way to add the data.

I cannot access the single ingredient association's amount using

ingredient = Ingredient.find(an_id)  

amount = @recipe.ingredient.amount

What is the method of select a single ingredients information that is stored in the join table?

Answer 1

Options :select is very useful in this case. For example

class Recipe
  has_many :ingredients, through: ingredients_recipes, select: "ingredinets.*, ingredients_recipes.some_additional_column as some_additional_column"
  ....

Then just call

Recipe.first.ingredients.first.some_additional_column # => profit!

You can use it in your scopes too.

Answer 2

You could @ingredient = @recipe.ingredients.first (if it is truly the only one, not sure how you could know this for sure.)

Assuming you already have the ingredient selected, as in a variable, you can set them like any other ActiveRecord object.

@ingredient = @recipe.ingredients.first
@ingredient.amount = 15 #or @ingredient.amount = params[:amount] (if you get the new value from POST)
@ingredient.unit = "Ounces"
@ingredient.save

If you just want to retrieve the attributes of each ingredient for a recipe and do something with them, like display them in some html tags in a view you could try this.

@ingredients = @recipe.ingredients
   @ingredients.each do |ingr|
    info = %w[amount, brand, id, recipe_id , unit].map do |attr|
      ingr.send(attr)
    end

at the end of each inner loop the array 'info' will be all the attributes of one ingredient.(assuming you include them in info array)

This is an example of a table you might use to store the associations, it should work with the rails :through => :recipe_ingedient method:

CREATE TABLE `recipe_ingredient` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `recipe_id` int(11) NOT NULL,
  `ingedient_id` int(11) NOT NULL,
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=59 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$

Answer 3

The join table is important for you, so it should have a name. You can call it recipe_ingredient, but a real name helps not to confuse the things mentally. (and it help us discussing about the problem)

Given the join table is called for example slot.

class Recipe
  has_many :slots
  has_many :ingredients, :through => :slots
  ..
class Slot
  belongs_to :recipe
  belongs_to :ingredient
  ...

You get a defined slot by calling

@slot = @recipe.slots.where(:ingredients_id => @ingredient).first 
@slot.amount = 20

For extra efficiency and to ensure that each combination exists only one time, you can define an unique compound index for slot to span across both recipe_id and ingredient_id

Answer 4

I don't see how such a thing could exist. If you have many associated objects, you need to specify which one you want. That's not so much a limitation of Rails as a limitation of... I don't know, set theory? Logic? You can get the one of the associated objects with .first (ingredient = @recipe.ingredients.first), but unless you also specify an order, there's no way to be certain which one it will be.