Can't lazy-load polymorphic relationships with different primary key columns

[fitztrev]

• Laravel Version: 5.6.26
• PHP Version: 7.2.6
• Database Driver & Version: MySQL 5.7.22

Description:

You cannot lazy-load models of a polymorphic relationship that have different primary key columns.

This functionality broke with #21310.

Scenario:

$comments = Comment::all();
$comments->load('commentable'); // this fails if you have the below schema

posts

post_id	title	body
111	Test Post	hello world

videos

video_id	title	video
222	Test Video	my-video.mov

comments

id	commentable_type	commentable_id	body
1	App\Post	111	"comment on a post"
2	App\Video	222	"comment on a video"

// This works (eager loading):
$comments = App\Comment::with('commentable')->get();

// This fails (lazy loading):
$comments = App\Comment::all();
$comments->load('commentable');
// > Results in "no such column: videos.post_id"

Steps To Reproduce:

I created a failing integration test to demonstrate.

## Pull down my fork
git clone -b polymorphic-diff-keys git@github.com:fitztrev/framework.git polymorphic-diff-keys

cd polymorphic-diff-keys
composer install

## Run the tests
## These 2 work
./vendor/bin/phpunit --filter testWithNoEagerOrLazyLoad
./vendor/bin/phpunit --filter testItCanEagerLoad

## This 1 fails
./vendor/bin/phpunit --filter testItCanLazyLoad

[staudenmeir]

#24063 has an explanation for this bug.

Currently, the only approach to fixing this is analyzing the backtrace. Since this is rather terrible, we are still looking for better ideas.