class Comment < ActiveRecord::Base
  belongs_to :post

  filterable_by :post_id, :user_id
  filterable_by :post_author_id do |value|
    joins(:posts).where(:'posts.author_id' => value)
  end
  filterable_by :only do |value, **opts|
    case value
    when 'mine'
      where(user_id: opts[:user_id]) if opts[:user_id]
    else
      all
    end
  end
end

Comment.filter_by(params[:filter], user_id: current_user.id)  # => ActiveRecord::Relation

Comment.filter_by({ 'post_id' => '1' })
# => WHERE post_id = 1

Comment.filter_by({ 'post_id' => ['1', '2'] })
# => WHERE post_id IN (1, 2)

Comment.filter_by({ 'post_id_not' => '3' })
# => WHERE post_id != 3

Comment.filter_by({ 'user_id' => '2', 'ignored' => '3' })
# => WHERE user_id = 2

Comment.filter_by({ 'only' => 'mine' }, user_id: 4)
# => WHERE user_id = 4

Comment.filter_by({ 'post_author_id' => '5' })
# => JOINS posts ON posts.id = comments.post_id WHERE posts.author_id = 5