Added a method to fluently set belongs-to relation

This method spun off from a discussion in IRC. What this enables is this:

$user->comments()->insert($comment_data)->article()->bind($article->id)

laravel/database/eloquent/relationships/belongs_to.php

@@ -111,4 +111,20 @@ class Belongs_To extends Relationship {
 		return $this->base->get_attribute($this->foreign);
 	}
 	
+	/**
+	* Bind an object over a belongs-to relation using its id.
+	*
+	* @return Eloquent
+	*/
+	
+	public function bind($id)
+	{
+		if((int) $this->foreign_value() === (int) $id)
+			return $this->base;
+
+		$this->base->fill(array($this->foreign => $id))->save();
+
+		return $this->base;
+	}
+
 }
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