has_one through association with condition

问题

I have 3 relevant tables/models. I would like to retrieve the user which has organized a party, given a Party record, a join table with a boolean organized column, and a users table.

My best attempt so far (this one makes the most sense to me after much fiddling). I've omitted the irrelevant columns.

class Party
  # party_id

  has_many   :parties_users
  has_many   :users, through: :parties_users, source: :user
  has_one    :organizer,
             -> { where organizer: true },
             through: :parties_users,
             source: :user

---

class PartiesUser
  # party_id
  # user_id
  # organized:bool

  belongs_to :party
  belongs_to :user

---

class User
  # user_id
  has_many : parties_users

---

The above setup raises the following error, which I honestly don't fully understand: ActiveRecord::HasOneThroughCantAssociateThroughCollection (Cannot have a has_one :through association 'Party#organizer' where the :through association 'Party#parties_users' is a collection. Specify a has_one or belongs_to association in the :through option instead.)

I know I can do this via an instance method, but given the frequency types of use, my app would massively benefit from having this as an association.

回答1:

As the error message says, you can't have a has_one through a has_many.

You could (instead) do it this way if the organizer flag is on the join record...

has_many :parties_users
has_many :users, through: :parties_users, source: :user
has_one :main_party_user, -> {where organizer: true}, class_name: 'PartiesUser'
has_one :organizer, through: :main_party_user, class_name: 'User'

来源：https://stackoverflow.com/questions/49366460/has-one-through-association-with-condition