在Rails 5.1 中，如何为has_many 和belongs_to 关联编写finder 方法？

Question

I'm using Rails 5.1. I have the following models ...

class Member < ApplicationRecord
  has_many :plans, :dependent => :nullify


class Plan < ApplicationRecord
  belongs_to :associate

I want to write a finder query that returns all Member objects with a Plan that has an "associate" with an ID of "1", "2", or "3". How do I do that? I tried writing a finder for only finding the ID, "1", but am getting this error upon evaluation ...

[8] pry(main)> Member.where(:plans => {:associate => {:id => "1"}}).count
   (0.7ms)  SELECT COUNT(*) FROM "Members" WHERE "associate"."id" IS NULL
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "associate"
LINE 1: SELECT COUNT(*) FROM "members" WHERE "associate"."id" ...
                                              ^
: SELECT COUNT(*) FROM "members" WHERE "associate"."id" IS NULL
from /Users/davea/.rvm/gems/ruby-2.5.1/gems/activerecord-5.2.2.1/lib/active_record/connection_adapters/postgresql_adapter.rb:611:in `async_exec'

Answer 1

You must first use joins to create an INNER JOIN query. After that you're able to access the join tables:

Member.joins(plans: :associate).where(associates: { id: 1 })