I am trying to create a test data for coverage. The field is a formula field and it has a master-detail relationship with another object, then I create a recode in the object. but how can I reference the field in the detail object? I am getting an error:

> 
> Field is not writeable
> 
> 
> 

How Can I do that?
The `PLS_Code__c` is a formula field and formulae are: `Positioner_Limit_Switch__r.Code__c`

```
    VWC_Positioner_LimitSwitch__c  PLSObj = new VWC_Positioner_LimitSwitch__c();

    PLSObj.Code__c = '2HPB';

    PLSObj.Description__c   = 'This is test data.';

    insert PLSObj;        

    VWC_PLS_Combination__c pls_Combination = new VWC_PLS_Combination__c();        

    pls_Combination.Price__c = 123;        

    pls_Combination.Positioner_Limit_Switch__r.Code__c = PLSObj.id;        
    insert pls_Combination;       
    instance.selectedPositioner = '2HPB';`

```