Android: how to use FragmentTransaction.add in a fragment -

so trying add fragment within fragment, told change fragment transaction variable go from:

fragmenttransaction ft = fm.begintransaction(); 

to

fragmenttransaction ft = getchildfragmentmanager().begintransaction(); 

however, not sure id use when call ft.add();

previously using android.r.id.content. there similar id can use fragment, or id should use able add new fragment within fragment.

edit:

activity_mainfrag:

<?xml version="1.0" encoding="utf-8"?> <linearlayout xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:tools="http://schemas.android.com/tools"     android:id="@+id/mainview"     android:layout_width="fill_parent"     android:layout_height="wrap_content"     android:background="#eeeeee"     android:orientation="vertical">      ...  </linearlayout> 

use add fragment:

    fragmenttransaction ft = getchildfragmentmanager().begintransaction();     ft.replace(r.id.container_parent, fragment, fragment.gettagtext());     ft.addtobackstack(fragment.gettagtext());     ft.commit();