How to model a real diode 1N4148?
The datasheets usually don't contain models for simple generic diodes. Some manufacturers may give out spice model for simulation purposes, but they are far more advanced than simply a resistor with a voltage source.

The model you happened to create is only a model, and it is specific to the environment it is in now (source voltage and impedance, load resistance, DC bias, AC signal). If that model is given to you or it is calculated from actual measurements then it should be correct.