TIL: Ecto 2.0 many_to_many

til.ex

# 1) Created User and Run models first (generated)
# 2) Created the UserRun model (generated) then removed all but Ecto.Schema
# 3) Corrected the UserRun model so that it looked like this:
defmodule Gatherto.UserRun do
  use Ecto.Schema
  
  schema "users_runs" do
    belongs_to :user, Gatherto.User  #this was VERY important! Before I had field :user_id, :binary_id
    belongs_to :run, Gatherto.Run    #this was VERY important! Before I had field :run_id, :binary_id
    timestamps()
  end
end

#From there I could - not sure if this is AOK (can I just use build_assoc ?)
dave = Repo.all(User) |> Repo.preload(:runs) |> List.first 
run = Repo.all(Run) |> List.first
changeset = Ecto.Changeset.change(dave) |> Ecto.Changeset.put_assoc(:runs, [run])
Repo.update(changeset)

#(This seemed not to work)
assoc = Ecto.build_assoc(dave, :runs, title: "Super cool", description: "Fast and long")
Repo.insert(assoc)
#In the above case a new Run gets created but is not associated with the user.

So, maybe the way to get all of this in a nicer place would be to expose User.changeset_with_run(user, run) ?