Extract mappedMorphTo function into a trait

Not sure if this is the best thing to do, it could also just be put on
the base Model class

src/Core/Support/MappedMorphToTrait.php

@@ -0,0 +1,39 @@
+<?php namespace Flarum\Core\Support;
+
+trait MappedMorphToTrait
+{
+    public function mappedMorphTo($classes, $name = null, $type = null, $id = null)
+    {
+        // If no name is provided, we will use the backtrace to get the function name
+        // since that is most likely the name of the polymorphic interface. We can
+        // use that to get both the class and foreign key that will be utilized.
+        if (is_null($name)) {
+            list(, $caller) = debug_backtrace(false, 2);
+
+            $name = snake_case($caller['function']);
+        }
+
+        list($type, $id) = $this->getMorphs($name, $type, $id);
+
+        // If the type value is null it is probably safe to assume we're eager loading
+        // the relationship. When that is the case we will pass in a dummy query as
+        // there are multiple types in the morph and we can't use single queries.
+        if (is_null($typeValue = $this->$type)) {
+            return new MappedMorphTo(
+                $this->newQuery(), $this, $id, null, $type, $name, $classes
+            );
+        }
+
+        // If we are not eager loading the relationship we will essentially treat this
+        // as a belongs-to style relationship since morph-to extends that class and
+        // we will pass in the appropriate values so that it behaves as expected.
+        else {
+            $class = $classes[$typeValue];
+            $instance = new $class;
+
+            return new MappedMorphTo(
+                $instance->newQuery(), $this, $id, $instance->getKeyName(), $type, $name, $classes
+            );
+        }
+    }
+}