Add custom key to display units in glossary with siunitx

Question

I decided to enrich my list of symbols by adding a unit column with a newly defined key "unit".

\glsaddkey
 {unit}
 {}
 {\glsentryunit}
 {\Glsentryunit}
 {\glsunit}
 {\Glsunit}
 {\GLSunit}

A new entry looks like

\newglossaryentry{insu}{
    type=symbols,
    name=thermal resistance,
    symbol={\ensuremath{R\mathrm{c}}},
    description={ratio of the temperature difference across an insulator and the heat flux for a particular material or assembly of materials},
    unit={\ensuremath\protect\si{\watt}},
}

and produces following error with the last curly brace, that closes the \newglossaryentry.

> ! Undefined control sequence.
\@glo@tmp ->\ensuremath \protect \si {\watt }

Moving the code inside the unit key, for testing purpose, to the key symbol works like a charm, but unfortunately not the intended place. Changes around the protect and ensuremath have no effect. The asterisk variant of \glsaddkey* doesn't help either.

What is the conflict between adding a new key to a glossary entry and the \si command?

Here a short MWE:

\documentclass{scrreprt}%
\usepackage{siunitx}
\usepackage{glossaries}

\glsaddkey
 {unit}
 {}
 {\glsentryunit}
 {\Glsentryunit}
 {\glsunit}
 {\Glsunit}
 {\GLSunit}

\makeglossaries

\newglossaryentry{test}{
 name=Test,
 unit=\si{\meter},
 description=sample description}

%-------------------------

\begin{document}

\printglossaries
\gls{test}

\end{document}

Answer 1

The glossaries package expands the input for all entries by default. This won't work with 'local' definitions for commands, as used by siunitx for unit commands (which can be something entirely different in other context). Possible solutions are to use \glsnoexpandfields:

\glsnoexpandfields
\newglossaryentry{test}{
 type=symbols,
 name=Test,
 unit=\si{\metre},
 description=sample description}

or to inhibit expansion of just the unit

\newglossaryentry{test}{
 type=symbols,
 name=Test,
 unit=\unexpanded{\si{\metre}},
 description=sample description}

or turn off expansion for the field

\glssetnoexpandfield{unit}