Access the $manyMethods property through static::$manyMethods

This allows adding additional $manyMethods when extending the model class
and still receive a correct guess for the relationship name.

Removed the name of guessBelongsToManyRelation from the list but still consider
it when analyzing the call stack. This makes $manyMethods more slim and easier
to override.

Removed findFirstMethodThatIsntRelation from $hasMany array, since it does not
appear anywhere in the code and seems to have been forgotten there.
It is also not really a name for a many-relationship method, so if it is needed,
i think there might be a better place for it.

src/Illuminate/Database/Eloquent/Concerns/HasRelationships.php

@@ -41,7 +41,6 @@ trait HasRelationships
      */
     public static $manyMethods = [
         'belongsToMany', 'morphToMany', 'morphedByMany',
-        'guessBelongsToManyRelation', 'findFirstMethodThatIsntRelation',
     ];
 
     /**
@@ -546,14 +545,19 @@ public function morphedByMany($related, $name, $table = null, $foreignPivotKey =
     }
 
     /**
-     * Get the relationship name of the belongs to many.
+     * Get the relationship name of the belongsToMany.
      *
-     * @return string
+     * @return string|null
      */
     protected function guessBelongsToManyRelation()
     {
+        // Search the call stack for the name of the relationship defined in the model
         $caller = Arr::first(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS), function ($trace) {
-            return ! in_array($trace['function'], Model::$manyMethods);
+            // Exclude names of the relationship methods and this method itself
+            return ! in_array(
+                $trace['function'],
+                array_merge(static::$manyMethods, ['guessBelongsToManyRelation'])
+            );
         });
 
         return ! is_null($caller) ? $caller['function'] : null;