[Tex/LaTex] How to cite properly cite an unpublished paper with correct note shown instead of year

Question

\documentclass[12pt]{report}
\usepackage{natbib}

I am using the natbib package for citations in my report. However I encounter a problem in citing an unpublished paper.

@unpublished{unpublishedkey,
author  = "Dummy, D1 D2",
title   = "Title",
note    = "unpublished"
}

When I use \citep{unpublishedkey}, I obtain

(Dummy, ????)

In the bibliography it shows,

Dummy, D.D. (????). Title. unpublished.

I try to add "unpublished" in the year field, but because of the truncation, it doesn't work properly:

@unpublished{unpublishedkey,
author  = "Dummy, D1 D2",
title   = "Title",
year    = "unpublished"
note    = "unpublished"
}

(Dummy, shed)

Dummy, D.D. (unpublished). Title

Isn't the year field optional in an unpublished bibtex entry, and how could I have the citation as the following instead?

(Dummy, unpublished)

Dummy, D.D. Title. unpublished.

Answer 1

You can assign a year, based on when you received the unpublished paper or use one of the following methods:

\begin{filecontents*}{\jobname.bib}
@unpublished{unpublishedkeyA,
author  = {Dummy, D1 D2},
title   = {Title},
year    = {N.D.},
note    = {unpublished},
}
@unpublished{unpublishedkeyB,
author  = {Dummy, D1 D2},
title   = {Title},
year    = {\ndd},
note    = {unpublished},
}
\end{filecontents*}

\documentclass[12pt]{report}
\usepackage{natbib}

\newcommand{\ndd}{Unpublished}

\begin{document}

Example of citation: \citep{unpublishedkeyA}

Another: \citet{unpublishedkeyA}

Example of citation: \citep{unpublishedkeyB}

Another: \citet{unpublishedkeyB}

\bibliographystyle{plainnat}
\bibliography{\jobname}

\end{document}

You can define \ndd any way you like.