Laravel comprueba si existe un modelo relacionado

In PHP 7.2+ no puedes usar count on the relation object, so there's no one-fits-all method for all relations. Use query method instead as @tremby provided below:

$model->relation()->exists()

generic solution working on all the relation types (pre php 7.2):

if (count($model->relation))
{
  // exists
}

This will work for every relation since dynamic properties return Model or Collection. Ambos implementan ArrayAccess.

Entonces es así:

single relations: hasOne / belongsTo / morphTo / morphOne

// no related model
$model->relation; // null
count($model->relation); // 0 evaluates to false

// there is one
$model->relation; // Eloquent Model
count($model->relation); // 1 evaluates to true

to-many relations: hasMany / belongsToMany / morphMany / morphToMany / morphedByMany

// no related collection
$model->relation; // Collection with 0 items evaluates to true
count($model->relation); // 0 evaluates to false

// there are related models
$model->relation; // Collection with 1 or more items, evaluates to true as well
count($model->relation); // int > 0 that evaluates to true

A Relation object passes unknown method calls through to an Eloquent query Builder, which is set up to only select the related objects. That Builder in turn passes unknown method calls through to su subyacente query Builder.

Esto significa que puede utilizar el exists() or count() methods directly from a relation object:

$model->relation()->exists(); // bool: true if there is at least one row
$model->relation()->count(); // int: number of related rows

Note the parentheses after relation: ->relation() is a function call (getting the relation object), as opposed to ->relation which a magic property getter set up for you by Laravel (getting the related object/objects).

Usando el count method on the relation object (that is, using the parentheses) will be much faster than doing $model->relation->count() or count($model->relation) (unless the relation has already been eager-loaded) since it runs a count query rather than pulling all of the data for any related objects from the database, just to count them. Likewise, using exists doesn't need to pull model data either.

Ambos exists() y count() work on all relation types I've tried, so at least belongsTo, hasOne, hasMany y belongsToMany.

Prefiero usar exists método:

RepairItem::find($id)->option()->exists()

to check if related model exists or not. It's working fine on Laravel 5.2

Después Php xnumx, The accepted answer won't work for all types of relationships.

Because depending of type the relationship, Eloquent will return a Collection, Model or Null. Y en Php xnumx count(null) lanzará un error.

So, to check if the relation exist you can use:

For relationships single: For example hasOne y belongsTo

if(!is_null($model->relation)) {
   ....
}

For relationships multiple: For Example: hasMany y belongsToMany

if ($model->relation->isNotEmpty()) {
   ....
}

I use for single relationships: hasOne, belongsTo y morphs

if($model->relation){ 
 ....
}

Because if condition is null, this will be false.

For multiple relationships: hasMany, belongsToMany y morphs

if ($model->relation->isNotEmpty()) {
   ....
}

Puede utilizar el relationLoaded method on the model object. This saved my bacon so hopefully it helps someone else. I was given this suggestion when I asked the same question on Laracasts.

As Hemerson Varela already said in Php 7.1 count(null) lanzará un error y hasOne devoluciones null if no row exists. Since you have a hasOnerelation I would use the empty método para comprobar:

$model = RepairItem::find($id);
if (!empty($temp = $request->input('option'))) {
   $option = $model->option;

   if(empty($option)){
      $option = $model->option()->create();
   }

   $option->someAttribute = temp;
   $option->save();
};

But this is superfluous. There is no need to check if the relation exists, to determine if you should do an update o un create call. Simply use the updateOrCreate method. This is equivalent to the above:

$model = RepairItem::find($id);
if (!empty($temp = $request->input('option'))) {  
   $model->option()
         ->updateOrCreate(['repair_item_id' => $model->id],
                          ['option' => $temp]);
}

Not sure if this has changed in Laravel 5, but the accepted answer using count($data->$relation) didn't work for me, as the very act of accessing the relation property caused it to be loaded.

In the end, a straightforward isset($data->$relation) hizo el truco para mí.

I had to completely refactor my code when I updated my PHP version to 7.2+ because of bad usage of the count($x) function. This is a real pain and its also extremely scary as there are hundreds usages, in different scenarios and there is no one rules fits all..

Rules I followed to refactor everything, examples:

$x = Auth::user()->posts->find(6); (check if user has a post id=6 using ->find())

[FAILS] if(count($x)) { return 'Found'; } 
[GOOD] if($x) { return 'Found'; }

$x = Auth::user()->profile->departments; (check if profile has some departments, there can have many departments)

[FAILS] if(count($x)) { return 'Found'; }
[GOOD] if($x->count()) { return 'Found'; }

$x = Auth::user()->profile->get(); (check if user has a profile after using a ->get())

[FAILS] if(count($x)) { return 'Found'; }
[GOOD] if($x->count()) { return 'Found'; }

Hopes this can help, even 5 years after the question has been asked, this stackoverflow post has helped me a lot!

If you use the model class and use Eloquent ORM, then create a new method and return bool data. like

public function hasPosts(): bool
{
    return $this->posts()->exists();
}

RelationLoaded method of Model class could be useful.

if ($this->relationLoaded('prices')) {
    return;
}