feature_and_milestone.rb

class Feature < ActiveRecord::Base
  has_one :project_item, as: :item, class_name: "ProjectItem"
end

class Milestone < ActiveRecord::Base
  has_one :project_item, as: :item, class_name: "ProjectItem"
end

problems.rb

Project.first.items           # fails, undefined method 'items'
Project.first.project_items   # no results ever
Project.first.features        # could not find the association :item in model Project
Project.first.milestones      # could not find the association :item in model Project

ProjectItem.first.feature     # undefined method 'feature'
ProjectItem.first.milestone   # undefined method 'milestone'

Feature.first.project_item.project.name # works, but expected Feature.first.item.project.name

project.rb

class Project < ActiveRecord::Base
  has_many :project_items, as: :item, class_name: "ProjectItem", dependent: :destroy
  has_many :features, through: :item, class_name: "ProjectItem", dependent: :destroy
  has_many :milestones, through: :item, class_name: "ProjectItem", dependent: :destroy
end

project_item.rb

class ProjectItem < ActiveRecord::Base
  belongs_to :item, touch: true, polymorphic: true
  belongs_to :project, touch: true
end

schema.rb

  create_table "project_items", force: true do |t|
    t.integer  "project_id"
    t.integer  "item_id"
    t.string   "item_type"
    t.integer  "position"
    t.datetime "created_at"
    t.datetime "updated_at"
  end
  
  create_table "projects", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end
  
  create_table "features", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end  

  create_table "milestones", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end