This gave me a little bit of grief.

We are using a slightly different name. A captain of a boat is necessarily a user. But we want to call him captain (or the proverbial ‘skipper’ - this is slang for captain, for those who are not native English speakers by the way).

Hence we have this sort of relationship:

class Boat < ApplicationRecord
  has_many :boat_captains
  has_many :captains, through: :boat_captains, source: :user
end

class BoatCaptain < ApplicationRecord
  belongs_to :boat
  belongs_to :user
end

class User
  has_many :boat_captains
  has_many :captains,  through: :boat_captains, source: :user
end

And rails does the rest.

A quick cheat sheet for us all!