follow.rb

class Follow < ActiveRecord::Base
  belongs_to :follower, class_name: 'User'
  belongs_to :followee, class_name: 'User'
end

post.rb

class Post < ActiveRecord::Base
  belongs_to :user
end

post_policy.rb

PostPolicy = Struct.new(:user, :post) do
  self::Scope = Struct.new(:user, :scope) do
    def resolve
        scope.joins(:user)
             .where(users[:private_profile].eq(false)
                    .or(posts[:user_id].eq(user.id))
                    .or(posts[:user_id].in(followers)))
      end
    end

    def followers
      follows.where(follows[:follower_id].eq(user.id))
        .where(follows[:approved].eq(true)).project(:followee_id)
    end

    def follows
      Arel::Table.new(:follows)
    end

    def users
      Arel::Table.new(:users)
    end

    def posts
      Arel::Table.new(:posts)
    end
  end
end

user.rb

class User < ActiveRecord::Base
  #has a private_profile flag
end