How to number labeled equations with different files?

Question

I have a file main.tex

\documentclass{article}

\usepackage{xr-hyper}
\usepackage{hyperref}
\usepackage{mathtools}
\mathtoolsset{showonlyrefs}
\begin{document}

\begin{equation}\label{sec:mytitle}
1 + 1 = 2
\end{equation}

\end{document}

and ref.tex

\documentclass{article}

\usepackage{xr-hyper}
\usepackage[colorlinks]{hyperref}
\hypersetup{
    colorlinks,
    linkcolor={blue},
    filecolor={red}, %<----
    urlcolor={blue},
    citecolor={blue}
}


\externaldocument[ex:]{external}

\begin{document}

\section{My section}\label{sec:mysection}

Local ref: \ref{sec:mysection}, external ref:  \ref{ex:sec:mytitle} 

\end{document}

I use showonlyrefs option in main.tex to minimize the numbering. However, I may have some equations that I will only call in ref.tex. These equations will not be numbered in main.tex by showonlyrefs. Is there any way I could overcome this?

Answer 1

Have a look at this post by Overleaf: Cross referencing with the xr package in Overleaf & xr package

Answer 2

Simply add \noeqref{sec:my_title} in main.tex.

Indeed, as the comment by @daleif points out, the documentation for the mathtools package discusses the \noeqref, which is similar to \nocite for bibliography items.