Phoenix form many_to_many

So I am trying to figure out how to work with Forms + Many to many relationships.

Lets say I have the following

schema "tour" do
    field :name, :string
    many_to_many(
      :city,
      City,
      join_through: "tour_cities",
      on_replace: :delete
    )
end

schema "city" do
    field :name, :string
end

And lets assume my tour_controller->create method
receives a params map like the following:

%{“city” => 1, “name” => “Best tour in the world”}

City is supplied via a dropdown in the form the
contents populated via a query on the City table.
1 is the ID of the city I want to relate to the tour I am about to create.

How do I make this work from the perspective of the
controller/changeset/whatever?

I have experimented with put_assoc, but that only
seems to work with a fully loaded Changeset. I
would ideally like to not have to hit the database
again to retrieve full changesets - when all I really
need is the ID of the City.

What I have made work:

put_assoc(:cities, new_tour, [%City{...fully loaded}, %City{...fully loaded}])

What doesn’t work:

put_assoc(:cities, new_tour, [%City{id: 2}, %City{id: 4}])

This example is simplified, eventually I want to
present the user with the option of adding multiple
cities at once in the form. But before I get to that,
I am trying to keep the form simple while I get the
many to many relationship working.