User.first.friends

User.first.friendships

+---------+-----------+
| user_id | friend_id |
+---------+-----------+
| 1       | 2         |
| 1       | 3         |
+---------+-----------+

create_table "friendships", id: false, force: :cascade do |t|
    t.integer "user_id"
    t.integer "friend_id"
end

create_table "users", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

class Friendship < ApplicationRecord
  belongs_to :user, foreign_key: "user_id", class_name: "User"
  belongs_to :friend, foreign_key: "friend_id", class_name: "User"
end

class User < ApplicationRecord
  has_many :friendships, foreign_key: "user_id", class_name: "Friendship"
  has_many :friends, through: :friendships
end