public virtual void EnsureField(Expression <Func <T, object> > selector)
{
// get proprerty name
var memberAccessor = new MemberAccessVisitor();
string propName = memberAccessor.GetMemberName(selector);
var prop = _entityType.GetProperty(propName);
var fieldType = FieldMapper.ToFieldType(prop);
EnsureFieldImpl(fieldType);
}