allow BelongsTo relations to return defaults

this implementation was copied and tweaked from the `HasOne`
relationship.

src/Illuminate/Database/Eloquent/Relations/BelongsTo.php

@@ -8,6 +8,15 @@
 
 class BelongsTo extends Relation
 {
+    /**
+     * Indicates if a default model instance should be used.
+     *
+     * Alternatively, may be a Closure or array.
+     *
+     * @var \Closure|array|bool
+     */
+    protected $withDefault;
+
     /**
      * The child model instance of the relation.
      */
@@ -72,7 +81,7 @@ public function __construct(Builder $query, Model $child, $foreignKey, $ownerKey
      */
     public function getResults()
     {
-        return $this->query->first();
+        return $this->query->first() ?: $this->getDefaultFor($this->parent);
     }
 
     /**
@@ -149,12 +158,39 @@ protected function getEagerModelKeys(array $models)
     public function initRelation(array $models, $relation)
     {
         foreach ($models as $model) {
-            $model->setRelation($relation, null);
+            $model->setRelation($relation, $this->getDefaultFor($model));
         }
 
         return $models;
     }
 
+    /**
+     * Get the default value for this relation.
+     *
+     * @param  \Illuminate\Database\Eloquent\Model  $model
+     * @return \Illuminate\Database\Eloquent\Model|null
+     */
+    protected function getDefaultFor(Model $model)
+    {
+        //return nothing if the user doesn't say to
+        if (! $this->withDefault) {
+            return;
+        }
+
+        //no need to set any attributes on this
+        $instance = $this->related->newInstance();
+
+        if (is_callable($this->withDefault)) {
+            return call_user_func($this->withDefault, $instance) ?: $instance;
+        }
+
+        if (is_array($this->withDefault)) {
+            $instance->forceFill($this->withDefault);
+        }
+
+        return $instance;
+    }
+
     /**
      * Match the eagerly loaded results to their parents.
      *
@@ -342,4 +378,17 @@ public function getRelation()
     {
         return $this->relation;
     }
+
+    /**
+     * Return a new model instance in case the relationship does not exist.
+     *
+     * @param  \Closure|array|bool  $callback
+     * @return $this
+     */
+    public function withDefault($callback = true)
+    {
+        $this->withDefault = $callback;
+
+        return $this;
+    }
 }

tests/Database/DatabaseEloquentBelongsToTest.php

@@ -9,11 +9,60 @@
 
 class DatabaseEloquentBelongsToTest extends TestCase
 {
+    protected $builder;
+
+    protected $related;
+
     public function tearDown()
     {
         m::close();
     }
 
+    public function testBelongsToWithDefault()
+    {
+        $relation = $this->getRelation()->withDefault(); //belongsTo relationships
+
+        $this->builder->shouldReceive('first')->once()->andReturnNull();
+
+        $newModel = new EloquentBelongsToModelStub();  //ie Blog
+
+        $this->related->shouldReceive('newInstance')->once()->andReturn($newModel);
+
+        $this->assertSame($newModel, $relation->getResults());
+    }
+
+    public function testBelongsToWithDynamicDefault()
+    {
+        $relation = $this->getRelation()->withDefault(function ($newModel) {
+            $newModel->username = 'taylor';
+        });
+
+        $this->builder->shouldReceive('first')->once()->andReturnNull();
+
+        $newModel = new EloquentBelongsToModelStub();
+
+        $this->related->shouldReceive('newInstance')->once()->andReturn($newModel);
+
+        $this->assertSame($newModel, $relation->getResults());
+
+        $this->assertSame('taylor', $newModel->username);
+    }
+
+    public function testBelongsToWithArrayDefault()
+    {
+        $relation = $this->getRelation()->withDefault(['username' => 'taylor']);
+
+        $this->builder->shouldReceive('first')->once()->andReturnNull();
+
+        $newModel = new EloquentBelongsToModelStub();
+
+        $this->related->shouldReceive('newInstance')->once()->andReturn($newModel);
+
+        $this->assertSame($newModel, $relation->getResults());
+
+        $this->assertSame('taylor', $newModel->username);
+    }
+
     public function testUpdateMethodRetrievesModelAndUpdates()
     {
         $relation = $this->getRelation();
@@ -126,18 +175,18 @@ public function testDefaultEagerConstraintsWhenNotIncrementing()
 
     protected function getRelation($parent = null, $incrementing = true, $keyType = 'int')
     {
-        $builder = m::mock('Illuminate\Database\Eloquent\Builder');
-        $builder->shouldReceive('where')->with('relation.id', '=', 'foreign.value');
-        $related = m::mock('Illuminate\Database\Eloquent\Model');
-        $related->incrementing = $incrementing;
-        $related->shouldReceive('getKeyType')->andReturn($keyType);
-        $related->shouldReceive('getIncrementing')->andReturn($incrementing);
-        $related->shouldReceive('getKeyName')->andReturn('id');
-        $related->shouldReceive('getTable')->andReturn('relation');
-        $builder->shouldReceive('getModel')->andReturn($related);
+        $this->builder = m::mock('Illuminate\Database\Eloquent\Builder');
+        $this->builder->shouldReceive('where')->with('relation.id', '=', 'foreign.value');
+        $this->related = m::mock('Illuminate\Database\Eloquent\Model');
+        $this->related->incrementing = $incrementing;
+        $this->related->shouldReceive('getKeyType')->andReturn($keyType);
+        $this->related->shouldReceive('getIncrementing')->andReturn($incrementing);
+        $this->related->shouldReceive('getKeyName')->andReturn('id');
+        $this->related->shouldReceive('getTable')->andReturn('relation');
+        $this->builder->shouldReceive('getModel')->andReturn($this->related);
         $parent = $parent ?: new EloquentBelongsToModelStub;
 
-        return new BelongsTo($builder, $parent, 'foreign_key', 'id', 'relation');
+        return new BelongsTo($this->builder, $parent, 'foreign_key', 'id', 'relation');
     }
 }