schema "users" do
    field :name, :string
    field :email, :string
    field :active, :boolean, default: true
    has_many :products, TestExAdmin.Product
    has_many :noids, TestExAdmin.Noid
    many_to_many :roles, TestExAdmin.Role, join_through: TestExAdmin.UserRole
  end

<div class="form-group">
       <label class="col-sm-2 control-label" for="user_roles">Roles</label>
       <div class="col-sm-10">
          <input name="user[roles][]" type="hidden" value="">
          <div class="checkbox"><label><input type="checkbox" name="user[roles][1]">role1</label></div>
       </div>
    </div>

def changeset(model, params \ %{}) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> cast_assoc(:noids, required: false)
    |> cast_assoc(:products, required: false)
    |> cast_assoc(:roles, required: false)
  end

%{email: "test@example.com", name: "Cory",
  products: %{"1481120458618": %{_destroy: "0", price: "13.00",
      title: "A product title"}}, roles: %{"1": "on"}}