Medium Article: Building Many-to-Many Associations with cast_assoc and Nested Forms in Phoenix and Ecto

schemas.ex

defmodule App.Account do
  schema "accounts" do
    field :name, :string
    has_many :memberships, App.Membership
    has_many :users, through: [:memberships, :user]
  end
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:name])
    |> validate_required([:name])
  end  
end

defmodule App.User do
  schema "users" do
    field :email, :string
    has_many :memberships, App.Membership
    has_many :accounts, through: [:memberships, :account]
  end
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:email])
    |> validate_required([:email])
    |> unique_constraint(:email)
  end  
end

defmodule App.Membership do
  schema "memberships" do
    belongs_to :account, App.Account
    belongs_to :user, App.User
  end
end