Insert equation labels inside a tikz(cd) diagram

Question

I have the following commuting diagram and I want to insert equation labels as (phantom) edge labels inside the diagram (the label here doesn't work, it's pseudocode):

\begin{tikzcd}
      A \arrow["h",dr] \arrow[dd,"j"'] \arrow["f",rr] & \arrow[d,phantom, "\label{eq:foo}" description] & B\arrow[dl, "g"] \\
      \arrow[r,phantom, "\label{eq:bar}" description]& C \\
      D\arrow[ru,"k"]
    \end{tikzcd}

And I want it to visually look like this:

Is there a way to do this? Basically I want to refer to different parts of the diagram commuting using different equation references.

Answer 1

The easiest solution would be where the tikzcd is using an unnumbered display math environment, i.e. equation* or \[ … \] and we will just place nodes with the equation number in it.

This seems to be compatible with hyperref but needs adjustments for cleveref (and then we might be able to introduce a separate category for those references as well).

All those macros are amsmath internals:

• \@eqnwstrue toggles the output of equation numbers inside math environments that don't have one for
• \make@display@tag which prints the actual equation number and
• \ltx@label is the original \label macro before amsmath added its own changes.

Code

\documentclass{article}
\usepackage{tikz, amsmath}
\usetikzlibrary{cd}
\makeatletter
\tikzcdset{
  eq node/.style={
    commutative diagrams/math mode=false, anchor=center},
  eq/.style={
    phantom,
    /tikz/every to/.append style={
      edge node={node[commutative diagrams/eq node]
        {\@eqnswtrue\make@display@tag\ltx@label{#1}}}}}}
\makeatother
\begin{document}
\[
\begin{tikzcd}
A       \arrow["h", dr] \arrow[dd,"j"'] \arrow["f",rr]
  &     \arrow[d, eq=eq:foo]
    & B \arrow[dl, "g"] \\
        \arrow[r, eq=eq:bar]
  & C \\
D       \arrow[ru,"k"]
\end{tikzcd}
\]
Equations \ref{eq:foo} and \eqref{eq:bar}.
\end{document}

Output