如何根据关联记录对记录进行排序

Question

class Conversation < ApplicationRecord
  has_many :messages, dependent: :destroy
  belongs_to :organization

  has_one :latest_message, -> {
    order(created_at: :desc)
  }, class_name: "Message", foreign_key: "conversation_id"
end

class Message < ApplicationRecord
  belongs_to :conversation
end

How do I order conversation list with the latest_message created_at time.

I tried

conversations.order("latest_message.created_at desc").to_a

but I'm getting the error

ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "latest_message"
12:33:07 web.1       | LINE 1: ...E "conversations"."organization_id" = $1 ORDER BY latest_mes...
12:33:07 web.1       |                                                              ^
12:33:07 web.1       | 
12:33:07 web.1       | from /home/rafi/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activerecord-6.0.2.2/lib/active_record/connection_adapters/postgresql_adapter.rb:672:in `exec_params'
12:33:07 web.1       | Caused by PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "latest_message"
12:33:07 web.1       | LINE 1: ...E "conversations"."organization_id" = $1 ORDER BY latest_mes...
12:33:07 web.1       |                                                              ^

possibly because the latest message is not a present in DB; but only in the model.

Answer 1

In ActiveRecord the arguments to .where , .group and .order need to be the actual table names and not associations as they generate SQL.

You don't really need that association at all though:

Conversation.joins(:messages)
            .group(:id) 
            .order("MAX(messages.created_at) DESC")

You may need to use .group('conversations.id') on some drivers like TinyTDS that will produce an ambiguous GROUP BY id query instead of GROUP BY "conversations"."id" . If you want to include conversations that don't have any messages use left_joins :

Conversation.left_joins(:messages)
            .group(:id) 
            .order("MAX(messages.created_at) DESC")