Issue with setting primary in MVG

Hi All,
Following is the piece of code.

BusObj = TheApplication().GetBusObject(BusObjectName);
pBC = BusObj.GetBusComp(ParentBusCompName);
pBC.SetViewMode(AllView);
pBC.ClearToQuery();
pBC.SetSearchSpec("Id", ObjectId);
pBC.ExecuteQuery(ForwardOnly);
if(pBC.FirstRecord())
{
    MVGBC = pBC.GetMVGBusComp(MVGField);
    ChildBC = MVGBusComp.GetAssocBusComp();
    ChildBC.SetViewMode(AllView);
    ChildBC.ClearToQuery();
    ChildBC.SetSearchExpr(SearchSpec);
    ChildBC.ExecuteQuery(ForwardOnly);
    if(ChildBC.FirstRecord())
    {
        ChildBC.Associate(NewAfter);
    }
}

Here I'm just trying to associate a record in the MVG applet through scripting.
Details of the MVL:

Auto Primary - Default
Check No Match - N
Primary Id Field - <primary id field in parent buscomp>
Source Field - <empty>
Use Primary Join - Y