Elixir ecto 2 create many_to_many association

Question

How can I make a many to many relation with ecto 2? As an example app I want to create a Post which can be in multiple categories. The categories already exist. For example

Answer 1

After a good night of sleep and some digging in the ecto unit tests i have found a partial answer. The right function to call is Ecto.Changeset.put_assoc. It returns a changeset. The rest of the question is on the bottom of this reply.

def run_insert_1 do
  c1 = Repo.get!(Category, 1)
  c2 = %Category{name: "cat 2"}

  # Inserting
  changeset =
    %Post{title: "1"}
    |> Ecto.Changeset.change
    |> Ecto.Changeset.put_assoc(:categories, [c1, c2])
  post = Repo.insert!(changeset)
  IO.inspect post
end

def run_insert_2 do
  c1 = Repo.insert! %Category{name: "cat 1"}
  c2 = %Category{name: "cat 2"}

  # Inserting
  changeset =
    %Post{title: "1"}
    |> Ecto.Changeset.change
    |> Ecto.Changeset.put_assoc(:categories, [c1, c2])
  post = Repo.insert!(changeset)
  IO.inspect post
end

def run_update do
  c1 = Repo.insert! %Category{name: "cat update"}
  c2 = %Category{name: "cat 2"}
  post = Repo.get!(Post, 1) |> Repo.preload(:categories)
  # Updating
  changeset =
    post
    |> Ecto.Changeset.change
    |> Ecto.Changeset.put_assoc(:categories, [c1])
  post = Repo.update!(changeset)
  IO.inspect post
end

It is a partial solution, because if i want to update the related categories (Post already has a list of related categories) I have to remove and then save the empty list of categories first. Is it possible to do this in one go?

def run_update_2 do
  c2 = Repo.get!(Tag, 2)
  # Assumes Post 1 already has  a few categories in it (for example after
  # running run_update()
  post = Repo.get!(Post, 1) |> Repo.preload(:categories)

  # Remove and add again
  changeset =
    post
    |> Ecto.Changeset.change
    |> Ecto.Changeset.put_assoc(:categories, [])
  IO.inspect changeset
  post = Repo.update!(changeset)

  changeset =
    post
    |> Ecto.Changeset.change
    |> Ecto.Changeset.put_assoc(:categories, [c2])

  post = Repo.update!(changeset)
  IO.inspect post
end