Define certain letter in math-mode to be always non-italic - using unicode-math in lualatex

Question

I once had this question a while ago, asking about how to set the letter j always to be non-italic j in math mode without using \mathrm.

The provided solution was:

\DeclareMathSymbol{j}{\mathalpha}{operators}{`j}

However, since I changed to lualatex and unicode-math the above line has either

no effect:

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec}          
\usepackage{unicode-math}

\DeclareMathSymbol{j}{\mathalpha}{operators}{`j}

\begin{document}

\begin{equation}
j ~~\text{is not}~~ \mathrm{j}
\end{equation}

\end{document}

or in case I define e.g. MinionPro and MnSymbol as my math fonts, the j

disappears completely:

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec}          
\usepackage{unicode-math}

\setmainfont{Minion Pro}
\setmathfont{MnSymbol}
\setmathfont[range=\mathup/{num,latin,Latin,greek,Greek}]{Minion Pro}
\setmathfont[range=\mathbfup/{num,latin,Latin,greek,Greek}]{MinionPro-Bold}
\setmathfont[range=\mathit/{num,latin,Latin,greek,Greek}]{MinionPro-It}
\setmathfont[range=\mathbfit/{num,latin,Latin,greek,Greek}]{MinionPro-BoldIt}
\setmathfont[range=\mathscr,StylisticSet={1}]{XITS Math}
\setmathrm{Minion Pro}

\DeclareMathSymbol{j}{\mathalpha}{operators}{`j}
  
\begin{document}

\begin{equation}
j ~~\text{is not}~~ \mathrm{j}
\end{equation}
But actually $j$ is not there at all.

\end{document}

I tried to declare the unicode character j separately, which didn't worked out.

\setmathfont[range={"006A} ]{Minion Pro}

It made appear the j again, but it was still italic.

---

Any new solutions for the old problem in case unicode-math is used?

I hope it was the appropriate action to ask a new question. I didn't wanted to alter the old question as it would have made the answer invalid.

---

Edit:

Not knowing that it would be important, I also need to include the amssymb package, which together with egreg's solution gives the following result:

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}            
    
\usepackage{fontspec}          
\usepackage{unicode-math}
\usepackage{etoolbox}

\setmainfont{Minion Pro}
\setmathfont{MnSymbol}
\setmathfont[range=\mathup/{num,latin,Latin,greek,Greek}]{Minion Pro}
\setmathfont[range=\mathbfup/{num,latin,Latin,greek,Greek}]{MinionPro-Bold}
\setmathfont[range=\mathit/{num,latin,Latin,greek,Greek}]{MinionPro-It}
\setmathfont[range=\mathbfit/{num,latin,Latin,greek,Greek}]{MinionPro-BoldIt}
\setmathfont[range=\mathscr,StylisticSet={1}]{XITS Math}
\setmathrm{Minion Pro}

\AfterEndPreamble{ %
\Umathcode`j="0"4"6A
}


\begin{document}

\begin{equation}
j ~~\text{is not}~~ \mathrm{j}
\end{equation}
But actually $j$ is not there at all.

\end{document}

Answer 1

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec}          
\usepackage{unicode-math}

\AtBeginDocument{\Umathcode`j="0"0"6A }

\begin{document}




\begin{equation}
j ~~\text{is}~~ \mathrm{j}
\end{equation}

\end{document}

I don't have MinionPro. but here is your extended version using Arial (you want 6)

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}            

\usepackage{fontspec}          
\usepackage{unicode-math}
\usepackage{etoolbox}

\setmainfont{Arial}
\setmathfont{MnSymbol}
\setmathfont[range=\mathup/{num,latin,Latin,greek,Greek}]{Arial}
\setmathfont[range=\mathbfup/{num,latin,Latin,greek,Greek}]{Arial}
\setmathfont[range=\mathit/{num,latin,Latin,greek,Greek}]{Arial}
\setmathfont[range=\mathbfit/{num,latin,Latin,greek,Greek}]{Arial}
\setmathfont[range=\mathscr,StylisticSet={1}]{XITS Math}
\setmathrm{Arial}

\AfterEndPreamble{ %
\Umathcode`j="0"6"6A
}


\begin{document}
\showoutput
\begin{equation}
j ~~\text{is}~~ \mathrm{j}
\end{equation}
But actually $j$ is not there at all.

\end{document}

Answer 2

Use the proper family number for j:

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec}
\usepackage{unicode-math}

\setmainfont{Minion Pro}
\setmathfont{MnSymbol}
\setmathfont[range=\mathup/{num,latin,Latin,greek,Greek}]{Minion Pro} % this is number 4
\setmathfont[range=\mathbfup/{num,latin,Latin,greek,Greek}]{MinionPro-Bold}
\setmathfont[range=\mathit/{num,latin,Latin,greek,Greek}]{MinionPro-It}
\setmathfont[range=\mathbfit/{num,latin,Latin,greek,Greek}]{MinionPro-BoldIt}
\setmathfont[range=\mathscr,StylisticSet={1}]{XITS Math}

\AtBeginDocument{\Umathcode`j="0"4"6A }

\begin{document}

\begin{equation}
j \text{ is } \mathrm{j}\ne\mathbf{j}\ne\mathit{j}
\end{equation}

\end{document}