I have a Parent__c and Child__c object with Child records using a custom Controller Extension and Visualforce page for EDIT and NEW.

Testing this controller extension for existing Child__c records is straight forward..

@isTest
private static void saveChangesToExistingChildRecord() {
    
    // Setup
    Parent__c parent = insertParent();
    Child__c existingChild = insertChild(parent);
    
    // Execute
    ApexPages.StandardController stdCtrl = new ApexPages.StandardController(existingChild);
    Child_CtrlExt ctrlExt = new Child_CtrlExt(stdCtrl);

    PageReference customEditPage = Page.childEdit;
    customEditPage.getParameters().put('id', existingChild.Id);
    ctrlExt.save();

    // Verify
    ...
}

... but how can I do this for new records (meaning the page called for New)?

@isTest
private static void newChildIsSaved() {
    
    // Setup
    Parent__c parent = insertParent();
    Child__c existingChild = null;
    
    // Execute
    // ???
    ApexPages.StandardController stdCtrl = new ApexPages.StandardController(existingChild);
    Child_CtrlExt ctrlExt = new Child_CtrlExt(stdCtrl);

    PageReference customEditPage = Page.childEdit;
    // ???
    customEditPage.getParameters().put('id', existingChild.Id); 
    ctrlExt.save();

    // Verify
    ...
}

Passing around null values to mimic a not yet existing object does not seem to work. When I look at the page params I see a cryptic LKID param, which seems to pass information about the parent object.

I have a Parent__c and Child__c object with Child records using a custom Controller Extension and Visualforce page for EDIT and NEW.

Testing this controller extension for existing Child__c records is straight forward..

@isTest
private static void saveChangesToExistingChildRecord() {
    
    // Setup
    Parent__c parent = insertParent();
    Child__c existingChild = insertChild(parent);
    
    // Execute
    ApexPages.StandardController stdCtrl = new ApexPages.StandardController(existingChild);
    Child_CtrlExt ctrlExt = new Child_CtrlExt(stdCtrl);

    PageReference customEditPage = Page.childEdit;
    customEditPage.getParameters().put('id', existingChild.Id);
    ctrlExt.save();

    // Verify
    ...
}

... but how can I do this for new records (meaning the page called for New)?

@isTest
private static void newChildIsSaved() {
    
    // Setup
    Parent__c parent = insertParent();
    Child__c existingChild = null;
    
    // Execute
    // ???
    ApexPages.StandardController stdCtrl = new ApexPages.StandardController(existingChild);
    Child_CtrlExt ctrlExt = new Child_CtrlExt(stdCtrl);

    PageReference customEditPage = Page.childEdit;
    // ???
    customEditPage.getParameters().put('id', existingChild.Id); 
    ctrlExt.save();

    // Verify
    ...
}

Passing around null values to mimic a not yet existing object does not seem to work. When I look at the page params I see a cryptic LKID param, which seems to pass information about the parent object.