incompatible.rb

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  gem "rails"
  gem "sqlite3"
end

require "active_record"
require "logger"

ActiveRecord::Base.establish_connection(
  adapter: "sqlite3",
  database: ":memory:"
)
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :users do |t|
    t.datetime :discarded_at
  end

  create_table :events do |t|
    t.references :user
    t.datetime :discarded_at
  end

  create_table :attendances do |t|
    t.references :user
    t.references :event
  end

  create_table :approvals do |t|
    t.string :status
    t.references :user
    t.references :event
    t.datetime :discarded_at
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  scope :kept, -> { where(discarded_at: nil) }
end

class User < ApplicationRecord
  has_many :events
  has_many :approvals
  has_many :attendances
end

class Event < ApplicationRecord
  default_scope { distinct }

  belongs_to :user

  has_many :approvals
  has_many :attendances

  scope :approved,
        -> {
          joins(:approvals).where(approvals: { status: :approved })
        }
  scope :attended_by,
        lambda { |user|
          joins(:attendances).where(attendances: { user_id: user.id })
        }
end

class Attendance < ApplicationRecord
  belongs_to :user
  belongs_to :event
end

class Approval < ApplicationRecord
  belongs_to :user
  belongs_to :event
end

user = User.create!

scope = Event.kept
scope = scope.approved.or(scope.where(user: user))
scope = scope.attended_by(user).or(scope.where(user: user))