Ecto many to Many Association working with preexisting-objects

Phoenix Forum Questions / Help
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1

Hello all,

I have the following question for a project I am developing.

I have two Schemas

  schema "recipes" do
    field :author, :string
    field :cooking_time_lower_limit, :integer
    field :cooking_time_upper_limit, :integer
    field :cousine, :string
    field :description, :string
 end

  schema "history_user_meals" do
    field :meal_datetime, :utc_datetime
    field :title, :string
    field :user_id, :id

    many_to_many :recipes, Recipe, join_through: RecipeUserMeal

    timestamps()
  end

Associated through a many to many relationship through the following schema

  schema "history_recipe_user_meals" do
    belongs_to :recipe, Recipe
    belongs_to :user_meal, UserMeal

    timestamps()
  end

What I want to do is through me api endpoint create/update/delete UserMeals and associate them with pre-existing Recipes .

Ideally what I would like to have would be the following

      valid_attrs = %{meal_datetime: ~U[2022-02-13 16:50:00Z], title: "some title", recipes: [%{recipe_id: recipe_0.id}, %{recipe_id: recipe.id}]}

      assert {:ok, %UserMeal{} = user_meal} = History.create_user_meal(valid_attrs)

and then for example I could update with a different set of recipe ids which would get rid of the old associations only keep the new ones.

The way I have come to believe that’s the way to go is to handle that by composition using a Multi.

and I was just wandering if there is a better or more elegant way so that for example Ecto can take care of the
associating and de-associating automatically through a changeset .

Tanks !

2

It looks like you want put_assoc. You create your changeset without the association and then pass it into here and then you perform your database operation.