Foreign Keys and Constraints: Enforcing Data Integrity

Database constraints prevent invalid data, even if a developer makes a mistake in code. Example: A post must always belong to a valid user. ## Add foreign key in migration ```php Schema::create('posts', function (Blueprint $table) { $table->id(); $table->foreignId('user_id')->constrained()->cascadeOnDelete(); $table->string('title'); $table->text('body'); $table->timestamps(); }); ``` This ensures: - user_id must exist in users table - if a user is deleted, their posts are deleted ## Graph: referential integrity ```mermaid flowchart LR A[users.id] --> B[posts.user_id] B --> C[DB Enforces Valid Link] ``` In the next tutorial, we will build an audit trail using events and model observers.