TikzCd: Midway label of a diagonal edge

Question

I have the following problem. I want to label the edge of A and B with a letter f. The thing is, if I use

\draw [->] (v1) -- node[midway]{$f$} (v2);

I get this:

And if I use

\draw [->] (v1) -- node[midway,above]{$f$} (v2);

I get this:

I want "f" to be at the place of the star * - in some positive distance perpendicular to the centre of A,B. But "midway" gives me f in the center of the line and "midway, above" gives me f that is little bit to the right. Similarly, "midway, left" gives me f that is below the star.

Is there a simple way to have f in the place of * ?

EDIT: My code:

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{tikz-cd}
\begin{document}
\begin{tikzpicture}
\node (v2) at (-1,2) {$B$};
\node (v1) at (-2.5,0.5) {$A$};
\draw [->] (v1) -- node[midway,above]{$f$} (v2);
\node at (-2,1.4) {*};
\end{tikzpicture}
\end{document}

EDIT2: Removed red crosses.

Answer 1

Welcome! There are two solutions. The first is that you can simply use the fill=white option to make sure the surrounding of the text painted white and then the text is drawn in black. The only caveat is that if you have a multi-coloured background (i.e., something with gradient), it will not look as nice.

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{tikz-cd}
\begin{document}
    \begin{tikzpicture}
        \node (v2) at (-1,2) {$B$};
        \node (v1) at (-2.5,0.5) {$A$};
        \draw [->] (v1) -- node[midway,fill=white]{$f$} (v2);
        \node at (-2,1.4) {*};
    \end{tikzpicture}
\end{document}

Another solution is to use xshift=-1em, yshift=1em options.

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{tikz-cd}
\begin{document}
    \begin{tikzpicture}
        \node (v2) at (-1,2) {$B$};
        \node (v1) at (-2.5,0.5) {$A$};
        \draw [->] (v1) -- node[midway, xshift=-1em, yshift=1em]{$f$} (v2);
        %\node at (-2,1.4) {*};
    \end{tikzpicture}
\end{document}

Answer 2

Since you tagged your question with tikz-cd, why don't you use it?

\documentclass{article}

\usepackage{tikz-cd}

\begin{document}
\[
\begin{tikzcd}
&B\\
A\ar[ur, "f"]\\
\end{tikzcd}
\]
\end{document}

Answer 3

My interpretation of the question is that you want the label be shifted along an orthogonal line of the connection. This is what the built in edge label provides you with. Otherwise, this can be achieved either by determining the slope by eyes or writing a style that measures the slope. Since the slope in this case is 45 degrees, the anchor must be at 45-90=-45. With above it will be at -90 (modulo upside down flips). With an edge label this is automatic. Nonetheless it might be worthwhile to recalculate this with calc to convince ourselves that it is so.

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}[Midlabel/.style={to path={
  let \p1=($(\tikztotarget)-(\tikztostart)$),
    \n1={atan2(\y1,\x1)}
  in (\tikztostart) -- (\tikztotarget) node[midway,circle,anchor=\n1-90]{#1}
  }}]
\begin{scope}[local bounding box=L]
 \node (v2) at (-1,2) {$B$};
 \node (v1) at (-2.5,0.5) {$A$};
 \draw [->] (v1) -- node[midway,above]{$f$} (v2);
 \node at (-2,1.4) {*};
\end{scope}  
\begin{scope}[local bounding box=M,xshift=4cm]
 \node (v2) at (-1,2) {$B$};
 \node (v1) at (-2.5,0.5) {$A$};
 \draw [->] (v1) -- node[midway,anchor=-45]{$f$} (v2);
 \node at (-2,1.4) {*};
\end{scope}  
\begin{scope}[local bounding box=R,xshift=8cm]
 \node (v2) at (-1,2) {$B$};
 \node (v1) at (-2.5,0.5) {$A$};
 \draw[->,Midlabel={$f$}] (v1) to (v2);
 \node at (-2,1.4) {*};
\end{scope} 
\begin{scope}[local bounding box=E,xshift=12cm]
 \node (v2) at (-1,2) {$B$};
 \node (v1) at (-2.5,0.5) {$A$};
 \draw[->] (v1) to[edge label={$f$}] (v2);
 \node at (-2,1.4) {*};
\end{scope} 
\path[font=\sffamily] (L.north) node[above]{original}
 (M.north) node[above]{node anchor adjusted by hand}
 (R.north) node[above]{with \texttt{Midlabel} style}
 (E.north) node[above]{with \texttt{edge label}};
\end{tikzpicture}
\end{document}

As tikz-cd uses edge labels, there this is automatic. The difference comes from the fact that in the Midlabel style the node shape is taken to be a circle, which as evenly spread angle anchors.

\documentclass{article}
\usepackage{tikz-cd}
\usetikzlibrary{calc}
\begin{document}
\tikzset{Midlabel/.style={to path={
  let \p1=($(\tikztotarget)-(\tikztostart)$),
    \n1={atan2(\y1,\x1)}
  in (\tikztostart) -- (\tikztotarget) node[midway,circle,anchor=\n1-90]{#1}
  }}}
\begin{tikzcd}
 & A \\
 B\arrow[ur,"f"] &
\end{tikzcd}

\begin{tikzcd}
 & A \\
 B\arrow[ur,"f"]\arrow[ur,Midlabel={$\scriptstyle f$}] &
\end{tikzcd}
\end{document}