Hello everyone!

Something happened at work today and it kinda got me off-guard because I was expecting different behavior from Ecto. What happened is: I was trying to insert a record with a given association that already existed in the database without casting the foreign key directly.

For example:

schema "post" do
  field :title, :string
  field :body, :string
end

schema "category" do
  field :name, :string
  field :description, :string
end

schema "category_post" do
  belongs_to :post, Post
  belongs_to :category, Category
  field :remarks, :string
end

def changeset(category_post, attrs) do
  post
  |> cast(attrs, [:remarks])
  |> cast_assoc(:post)
  |> cast_assoc(:category)
  |> validate_if_posts_has_the_right_amount_of_comments() # needs the retrieved post
  |> validate_if_category_has_the_right_amount_of_followers() # needs the retrieved category
end

And then, trying to create a “category_post” with its relationship:

def create_category_post(attrs, post_id, category_id) do
  post = Posts.get_post!(post_id)
  category = Categories.get_category!(category_id)

  attrs = Map.merge(attrs, %{category: category, post: post})
   
  %CategoryPost{}
  |> CategoryPost.changeset(attrs)
  |> Repo.insert()
end

Is this even possible? I noticed that for this case Ecto always tries to insert a new “post”/ “category” if I use cast_assoc/3.