Laravel belongs to relation, but with a combination of fields

A relation uses the parent id and in child a foreign id.

id would point to company_id  // whatever the foreign key is

Any other fields would be there to use as needed. Have you practiced some of the examples from:

https://laravel.com/docs/10.x/eloquent-relationships

To create a belongsTo relationship between the Customer and Company models based on the combination of the cnpj_base, cnpj_ordem, and cnpj_dv columns in the Company model, you can define the relationship in the Customer model as follows:

class Customer extends Model
{
    public function company()
    {
        return $this->belongsTo(Company::class, 'company_code', 'code');
    }
}

In the belongsTo method, the first argument is the related model name (Company::class), the second argument is the foreign key (company_code), and the third argument is the owner key (code).

Next, in the Company model, you can define an accessor to concatenate the cnpj_base, cnpj_ordem, and cnpj_dv columns into the code attribute:

class Company extends Model
{
    public function getCodeAttribute()
    {
        return $this->attributes['cnpj_base'] . str_pad($this->attributes['cnpj_ordem'], 4, '0', STR_PAD_LEFT) . $this->attributes['cnpj_dv'];
    }

    public function customers()
    {
        return $this->hasMany(Customer::class, 'company_code', 'code');
    }
}

In the getCodeAttribute method, we concatenate the cnpj_base, cnpj_ordem, and cnpj_dv columns into the code attribute.

Finally, you can retrieve the related Company model from a Customer instance as follows:

$customer = Customer::find(1);
$company = $customer->company;

This will return the Company model with the code attribute matching the company_code column in the Customer model.