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For the scenario, imagine posts has a foreign key user_id referencing users.id

public function up()
{
    Schema::create('posts', function(Blueprint $table) {
        $table->increments('id');
        $table->string('title');
        $table->text('body');
        $table->integer('user_id')->unsigned();
        $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
        $table->timestamps();
    });
}

running seeds with truncate() will trigger a constraint violation if foreign key checks is not disabled.

    Syntax error or access violation: 1701 Cannot truncate a table referenced in a foreign key constraint ...

The fix:

DatabaseSeeder.php

class DatabaseSeeder extends Seeder {

    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        Eloquent::unguard();

        //disable foreign key check for this connection before running seeders
        DB::statement('SET FOREIGN_KEY_CHECKS=0;');
            
        $this->call('UsersTableSeeder');
        $this->call('PostsTableSeeder');

        // supposed to only apply to a single connection and reset it's self
        // but I like to explicitly undo what I've done for clarity
        DB::statement('SET FOREIGN_KEY_CHECKS=1;');
        
    }

}

fix from here http://stackoverflow.com/questions/14666277/laravel-4-working-with-relationships-in-seeds

As pointed out in the comments at SO you wouldn't want to disable foreign key checks on a production app but you really don't want to run seed files in production either so obviously this is not intended for production but helpful in development.