Handling mapping table in laravel eloquent

In my application, there will be multiple investors tagged for single purchase entry. So on loading a purchase entry, I should get all the investors associated.

In my controller,

return response()->json(GoldPurchase::with('investors')->get());

Mapping table schema,

Schema::create('gold_purchase_investor', function (Blueprint $table) {
   $table->increments('id');
   $table->integer('investor_id')->unsigned();
   $table->integer('purchase_id')->unsigned();
   $table->timestamps();

   $table->foreign('investor_id')
        ->references('id')
        ->on('investors')
        ->onDelete('cascade');

   $table->foreign('purchase_id')
        ->references('id')
        ->on('gold_purchases')
        ->onDelete('cascade');
});

Purchase model,

class GoldPurchase extends Model
{
    public function investors() {
        return $this->hasMany('App\GoldPurchaseInvestor');
    }
}

Investor model,

class Investor extends Model
{
    protected $fillable = ['name', 'address', 'mobile', 'email'];

    public function purchases() {
        return $this->hasMany('App\GoldPurchase');
    }
}

PurchaseInvestor model,

class GoldPurchaseInvestor extends Model
{
    protected $table = 'gold_purchase_investor';

    public function purchase() {
        return $this->belongsTo('App\GoldPurchase');
    }

    public function investor() {
        return $this->belongsTo('App\Investor');
    }
}

With this, I am getting error,

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'gold_purchase_investor.gold_purchase_id' in 'where clause' (SQL: select * from `gold_purchase_investor` where `gold_purchase_investor`.`gold_purchase_id` in (1))

via Chebli Mohamed