Test REQUIRED_FIELD_MISSING even though field is set?

Question

@isTest
private class Test_ordinefermo {

    @isTest
    static void test_newAttachment(){
        test.startTest();
            Opportunity op= new Opportunity();
            op.Name= 'abc';
            op.StageName='Open';
            op.Trattativa__c = 'Automotive';
            op.CloseDate = date.valueOf('2017-11-12');
            op.Tipo_di_Trattativa__c = 'Cross Selling';
            insert op;
            file_ordine_fermo__c fofc = new file_ordine_fermo__c();
            fofc.Name = 'fofcdemo';
            fofc.Trattativa_Correlata__c =op.Id;
            insert fofc;
            ApexPages.StandardController std = new ApexPages.StandardController(fofc);
            ordinefermo ord= new ordinefermo(std);
            ord.newAttachment();
        test.stopTest();
    }
}

I have above test class testing a extention ordinefermo. when I try to test newAttachment() method is giving me

System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Trattativa_Correlata__c]: [Trattativa_Correlata__c]

But I have already populated Trattativa_Correlata__c field in my test class. When I try to deleting the statement ord.newAttachment(); the test case is being successful but I am falling behind code coverage as I am not testing the method.

Answer 1

Id for the opportunity is not passed to controller. Your method is getting id from apex page parameter. So you need to pass opportunity id to your page. Try this in your test method before calling newAttacment():

ApexPages.currentPage().getParameters().put('idopp',op.id);