[5.6] Allow eager-loading mixed relationships of polymorphic collection

[derekmd]

I've had this macro kicking around for awhile since the framework doesn't offer a way to avoid N+1 queries from morphTo() nested relationships. This adds Illuminate\Database\Eloquent\Collection@loadMorph() to allow nested eager loading by naming the unique relationship(s) for each class name.

Issue reported:

• Eager loading on polymophic modal's relations #15248

External Laravel community:

• Activities profile page N+1 issue laracasts/Lets-Build-a-Forum-in-Laravel#4
  • In response to https://laracasts.com/series/lets-build-a-forum-with-laravel/episodes/26
• http://derekmd.com/2017/06/eager-loading-laravel-polymorphic-relations/
  • My long-winded write-up about what this is solving.

Example Use Case

class ActivityFeed
{
    public function parentable()
    {
        // e.g., hydrate a Collection of Event, Photo, or Post instances.
        return $this->morphTo();
    }
}

$activities = ActivityFeed::with('parentable')
    ->get()
    ->loadMorph('parentable', [ // could instead be named loadMixed()? loadMorphed()?
        Event::class => 'calendar',
        Photo::class => 'tags',
        Post::class => ['author', 'commentsCount'],
    ]);

In an ideal world...

...this would be supported:

ActivityFeed::with([
    'parentable' => [
        Event::class => 'calendar',
        Photo::class => 'tags',
        Post::class => ['author', 'commentsCount'],
    ],
    'user:id,name',
    'zonda' => function ($query) {
        $query->ofSomeScope();
    },
])->get();

I really wanted to work this into the existing eager-load infrastructure:

• Illuminate\Database\Eloquent\Builder@with()
• Illuminate\Database\Eloquent\Collection@load()
• Illuminate\Database\Eloquent\Model@with (array object property)

However this is going to require a significant rewrite of the Eloquent internals that store an associative array of query constraint Closures.

Pagination

For better developer experience in controllers, an explicit AbstractPaginator@loadMorph() proxy is added to return the paginator. This allows it to be passed into the view or returned by an API response.

Then controllers don't have to awkwardly do:

$tags = Tag::with('taggable')->latest()->paginate();

$tags->getCollection()->loadMorph('taggable', [
    Post::class => 'category', 'comments',
    Video::class => 'likes',
]);

return view('tags.index')->with('tags', $tags);

but:

$tags = Tag::with('taggable')->latest()->paginate()->loadMorph('taggable', [
    Post::class => ['category', 'comments'],
    Video::class => 'likes',
]);

return view('tags.index')->with('tags', $tags);

[taylorotwell]

We don't support eager loading morphTo relationships already? I see the code in the framework to support it?

[derekmd]

This is allowing eager-loading from mixed classes that have relationships uncommon between each other. From the above example:

$activities = ActivityFeed::with([
    'parentable.calendar',      // Event@calendar() hasOne
    'parentable.tags',          // Photo@tags() hasMany, relation doesn't exist in Event class
    'parentable.author',        // Post@author() hasOne
    'parentable.commentsCount', // Post@commentsCount hasOne
])
->get();

Illuminate\Database\Eloquent\RelationNotFoundException: Call to undefined relationship [tags] on model [App\Models\Event].

I didn't see anything on https://laravel.com/docs/5.6/eloquent-relationships#eager-loading or in the source code that supports such eager loading?

[taylorotwell]

Gotcha.

[taylorotwell]

I'm fine if you want to add the paginator helper to this PR so I can review it all at once.

[sylouuu]

@derekmd is there any plan to work on your "ideal world" (which is amazing btw)?

This problem does not encourage developers to use polymorphic relation. :(

I have to process hundreds of polymorphic records and play with nested relations and the overload of queries is a nightmare.

Best