TikZ ER diagrams: really can't underline keys?

Question

I'm creating an ER diagram with the TikZ ER library. The manual warns that although key attributes are conventionally underlined, it is hard to implement in TeX and the library uses italics instead. That's fine for me but my teachers probably won't accept the assignment with non-standard notations.

That's why I'm asking the TeX wizard community for a working solution :) Questions that may be relevant are:

• Custom switches (such as for underlining)
• Use node text as argument for a macro

MWE from the manual:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{er}

\begin{document}
\begin{tikzpicture}
\node[entity] (sheep) at (0,0) {Sheep}
            child {node [key attribute] {name}};
\node[entity] (genome) at (2,0) {Genome};
\node[relationship] at (1,1.5) {has}
            edge (sheep)
            edge (genome);
\end{tikzpicture}
\end{document}

Answer 1

Assuming I understand the requirements correctly, it is possible to 'capture' the node contents in a box using execute at begin node and execute at end node and do something afterwards with it. This only possible with 'simple' node contents (i.e., basically just text).

\documentclass[border=0.125cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{er}
\newbox\underlinebox
\tikzset{
    key attribute/.append style={
        font=\rmfamily,
        execute at begin node={%
            \setbox\underlinebox=\hbox\bgroup
        },
        execute at end node={%
             \egroup\underline{\box\underlinebox}%
        }
    }
}
\begin{document}
\begin{tikzpicture}
\node[entity] (sheep) at (0,0) {Sheep}
            child {node [key attribute] {name}};
\node[entity] (genome) at (2,0) {Genome};
\node[relationship] at (1,1.5) {has}
            edge (sheep)
            edge (genome);
\end{tikzpicture}
\end{document}