使用ECTO和put_assoc/4实现多对多

defmodule A do
  use Ecto.Schema

  schema "a" do
    has_many :ab, B, join_through: "ab"
    # 其他字段定义
  end
end

defmodule B do
  use Ecto.Schema

  schema "b" do
    has_many :ab, A, join_through: "ab"
    # 其他字段定义
  end
end

defmodule CreateAB do
  use Ecto.Migration

  def change do
    create table(:ab) do
      add :a_id, references(:a, on_delete: :nothing)
      add :b_id, references(:b, on_delete: :nothing)
    end

    create index(:ab, [:a_id, :b_id], unique: true)
  end
end

a = Repo.get(A, a_id)
b = Repo.get(B, b_id)

changeset = A.changeset(a, %{ab: [b]})
a_with_b = Repo.update(changeset)

query = from a in A,
        join: ab in assoc(a, :ab),
        join: b in B, on: b.id == ab.b_id,
        where: a.id == ^a_id

Repo.all(query)