Add foreign key references

database/migrations/2017_11_01_165250_add_references.php

@@ -0,0 +1,40 @@
+<?php
+
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Migrations\Migration;
+
+class AddReferences extends Migration
+{
+    public function up()
+    {
+        Schema::table('topics', function (Blueprint $table) {
+
+            // 当 user_id 对应的 users 表数据被删除时，删除词条
+            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+        });
+
+        Schema::table('replies', function (Blueprint $table) {
+
+            // 当 user_id 对应的 users 表数据被删除时，删除此条数据
+            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+
+            // 当 topic_id 对应的 topics 表数据被删除时，删除此条数据
+            $table->foreign('topic_id')->references('id')->on('topics')->onDelete('cascade');
+        });
+    }
+
+    public function down()
+    {
+        Schema::table('topics', function (Blueprint $table) {
+            // 移除外键约束
+            $table->dropForeign(['user_id']);
+        });
+
+        Schema::table('replies', function (Blueprint $table) {
+            $table->dropForeign(['user_id']);
+            $table->dropForeign(['topic_id']);
+        });
+
+    }
+}