Laravel belongsTo foreignkey not working

I'm relatively new to Laravel and I am trying to make a forum using eloquent.

I used the make:auth command to make a users migration and created a threads migration using mysql workbench and a plugin to convert the ERD to a migration:

    Schema::create($this->set_schema_table, function (Blueprint $table) {
        $table->engine = 'InnoDB';
        $table->increments('id');
        $table->string('title', 45)->nullable();
        $table->text('description')->nullable();
        $table->timestamp('created_at')->nullable()->default(DB::raw('CURRENT_TIMESTAMP'));
        $table->timestamp('updated_at')->nullable()->default(DB::raw('CURRENT_TIMESTAMP'));
        $table->unsignedInteger('created_by');

        $table->index(["created_by"], 'fk_threads_users1_idx');


        $table->foreign('created_by', 'fk_threads_users1_idx')
            ->references('id')->on('users')
            ->onDelete('no action')
            ->onUpdate('no action');
    });

After that I created a model for threads and extended the user model to specify the relationship between the two:

class Thread extends Model
{
    // No fields are protected in the database
    protected $guarded = [];

    public function user(){
        return $this->belongsTo(User::class, 'created_by');
    }
}

and

class User extends Authenticatable
{

    public function threads(){
        return $this->hasMany(Thread::class);
    }

    public function publish(Thread $thread){
        $this->threads()->save($thread);
    }
}

At first this worked fine, but somehow after running php artisan cache:clear (or maybe something else may have caused the code to stop working, I am not sure) the store method in the thread controller gave me an error:

Column not found: 1054 Unknown column 'user_id' in 'field list' 
(SQL: insert into `threads` (`title`, `content`, `user_id`, `updated_at`, `created_at`) 
values (Thread 4, content, 17, 2017-11-23 11:16:24, 2017-11-23 11:16:24))`

As you can see it's trying to find the field "user_id" while I specified the foreign key should be "created_by" in the user method in the Thread class.

I'm pretty sure everything worked at fine at first. Does anyone know how to fix this?