Fix migration errors on form_submissions (#324)

* Fix migration errors on form_submissions

* Update migration approach

Author: ryanmitchell

database/migrations/updates/relate_form_submissions_by_handle.php.stub

@@ -9,6 +9,10 @@ use Statamic\Eloquent\Forms\SubmissionModel;
 return new class extends Migration {
     public function up()
     {
+        if (Schema::hasColumn($this->prefix('form_submissions'), 'form')) {
+            return;
+        }
+
         Schema::table($this->prefix('form_submissions'), function (Blueprint $table) {
             $table->string('form', 30)->nullable()->index()->after('id');
         });
@@ -24,7 +28,12 @@ return new class extends Migration {
             });
 
         Schema::table($this->prefix('form_submissions'), function (Blueprint $table) {
-            $table->dropForeign(['form_id']);
+            collect(Schema::getForeignKeys($this->prefix('form_submissions')))->each(function ($fk) use ($table) {
+                if (count($fk['columns']) == 1 && in_array('form_id', $fk['columns'])) {
+                    $table->dropForeign(['form_id']);
+                }
+            });
+
             $table->dropColumn('form_id');
         });
     }