Rails Query by specified primary key issue

output.log

-- create_table(:posts, {:force=>true})
D, [2015-02-04T09:40:42.602488 #22036] DEBUG -- :    (0.2ms)  CREATE TABLE "posts" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "system_id" integer) 
   -> 0.0041s
-- create_table(:comments, {:force=>true})
D, [2015-02-04T09:40:42.602859 #22036] DEBUG -- :    (0.1ms)  CREATE TABLE "comments" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "post_id" integer) 
   -> 0.0003s
Run options: --seed 15155

# Running:

D, [2015-02-04T09:40:42.621038 #22036] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-04T09:40:42.625989 #22036] DEBUG -- :   SQL (0.1ms)  INSERT INTO "posts" ("system_id") VALUES (?)  [["system_id", 23]]
D, [2015-02-04T09:40:42.626287 #22036] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-02-04T09:40:42.632388 #22036] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-04T09:40:42.632792 #22036] DEBUG -- :   SQL (0.0ms)  INSERT INTO "comments" DEFAULT VALUES
D, [2015-02-04T09:40:42.632980 #22036] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-02-04T09:40:42.633107 #22036] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-04T09:40:42.634920 #22036] DEBUG -- :   SQL (0.1ms)  UPDATE "comments" SET "post_id" = ? WHERE "comments"."id" = ?  [["post_id", 23], ["id", 1]]
D, [2015-02-04T09:40:42.635141 #22036] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-02-04T09:40:42.638313 #22036] DEBUG -- :   Comment Exists (0.1ms)  SELECT  1 AS one FROM "comments" WHERE "comments"."post_id" = 1 LIMIT 1
F

Finished in 0.020492s, 48.7997 runs/s, 48.7997 assertions/s.

  1) Failure:
BugTest#test_association_stuff [test_foreign_key.rb:46]:
Failed assertion, no message given.

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips

test.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true  do |t|
    t.integer :system_id
  end

  create_table :comments, force: true  do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments, primary_key: :system_id
end

class Comment < ActiveRecord::Base
  belongs_to :post, primary_key: :system_id
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create! system_id: 23
    post.comments << Comment.create!

    assert Comment.exists?(post: post)
  end
end