adding edge label to tikz and bayesnet library

Question

I want to generate similar graph as follows using bayesnet library but I do not know how I can add labels for edges

This is my code so far

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{tikz}
\usetikzlibrary{bayesnet}

\begin{document}
\begin{figure}
\begin{tikzpicture}
%Nodes
\node[latent,rectangle]    (X)      {$$} ; %
\node[latent,rectangle,left=of X, xshift=-2.5cm]    (Y)      {$$} ; %
%connect X,Y nodes
\draw (X)  --  (Y);
\draw (X)  --  (Y) node [midway,above](TextNode){$Z_k$};
\draw (X)  --  (Y) node [very near end,above](TextNode){$q(z_k)$};
\draw (X)  --  (Y) node [very near start,above](TextNode){$q(z_k)$};
\draw (X)  --  (Y) node [very near end,below](TextNode){$\nu(z_k)$};
\draw (X)  --  (Y) node [very near start,below](TextNode){$\nu(z_k)$};

\end{tikzpicture}
\end{figure}

\end{document}

How could I also generate this graph using this library?

Thanks

Answer 1

I've used arrows.meta TikZ library for the arrow tips, matrix to align the X's and the V's, fit to create the bigger node avoiding manual calculations, and decorations.markings to easily put the arrows.

The \cev macro is from this answer by egreg.

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx,accents}
% this code is from: https://tex.stackexchange.com/a/245346/101651
\makeatletter
\DeclareRobustCommand{\cev}[1]{%
  \mathpalette\do@cev{#1}%
}
\newcommand{\do@cev}[2]{%
  \fix@cev{#1}{+}%
  \reflectbox{$\m@th#1\vec{\reflectbox{$\fix@cev{#1}{-}\m@th#1#2\fix@cev{#1}{+}$}}$}%
  \fix@cev{#1}{-}%
}
\newcommand{\fix@cev}[2]{%
  \ifx#1\displaystyle
    \mkern#23mu
  \else
    \ifx#1\textstyle
      \mkern#23mu
    \else
      \ifx#1\scriptstyle
        \mkern#22mu
      \else
        \mkern#22mu
      \fi
    \fi
  \fi
}
\makeatother

\usepackage{tikz}
\usetikzlibrary{bayesnet, arrows.meta}
\usetikzlibrary{matrix, fit}
\usetikzlibrary{decorations.markings}
\tikzset{
    myarrow/.style={
        -Stealth,
        shorten >=8pt,
        shorten <=8pt,
    },
    mymatrix/.style={
        matrix of math nodes
    },
}
\begin{document}
\begin{figure}[htb]\centering
\begin{tikzpicture}
%Nodes
\node[latent,rectangle] (X) {}; 
\node[latent,rectangle,left=3.5cm of X] (Y) {};
%connect X,Y nodes
\draw (X)  --  (Y) 
    node [midway,above](TextNode){$Z_k$}
    node [very near end, above=6pt] (TextNodenw) {$q(z_k)$} 
    node [very near start,above=6pt](TextNodene){$q(z_k)$} 
    node [very near end,below=6pt](TextNodesw){$\vec{\nu}(z_k)$} 
    node [very near start,below=6pt](TextNodese){$\cev{\nu}(z_k)$}
    ;
    \draw[myarrow] (TextNodenw.south east) -- (TextNodenw.south west);
    \draw[myarrow] (TextNodene.south west) -- (TextNodene.south east);
    \draw[myarrow] (TextNodesw.north west) -- (TextNodesw.north east);
    \draw[myarrow] (TextNodese.north east) -- (TextNodese.north west);
\end{tikzpicture}
\caption{The first graph}
\end{figure}
\begin{figure}[htb]\centering
\begin{tikzpicture}
%Nodes
\node[latent,rectangle] (g) {$g$}; %
\matrix[mymatrix, left =8pt of g, inner xsep=1pt](mym){
    X_1\\
    \vdots\\[3pt]
    X_{n-1}\\
    };
\matrix[
    mymatrix, 
    above=40pt of g,
    column sep=1em,
    inner sep=1pt
    ](myv){
    V_1 & V_2 & \cdots & V_{r}\\
    };
\draw (g.east) -- ++(1,0)
    node[midway, above] (xn) {$X_n$}
    node[at end, inner sep=0pt](myc){};
\draw[-Stealth] ([yshift=-4pt]xn.south east) -- ([yshift=-4pt]xn.south west);
\draw[Stealth-] ([yshift=-8pt]xn.south east) -- ([yshift=-8pt]xn.south west);
\node[fit=(mym)(myc), draw, inner sep=0pt](bigsquare){};
\draw[
    postaction={decorate},
    decoration={markings,mark=at position 0.5 with {\draw[myarrow](0,-4pt)--(10pt,-4pt);}},
    ] (mym-1-1.south) -- (g.160);
\draw[
    postaction={decorate},
    decoration={markings,mark=at position 0.5 with {\draw[myarrow](0,4pt)--(10pt,4pt);}},
    ] ([yshift=-1pt]mym-3-1.north) -- (g.-160);
\draw[
    postaction={decorate},
    decoration={markings,mark=at position 0.4 with {\draw[myarrow](0,-4pt)--(10pt,-4pt);}},
    ] ([yshift=-1pt]myv-1-1.north east) -- (g.120);
\draw[
    postaction={decorate},
    decoration={markings,mark=at position 0.4 with {\draw[myarrow](0,4pt)--(10pt,4pt);}},
    ] ([yshift=-1pt]myv-1-2.north east) -- (g.90);
\draw[
    postaction={decorate},
    decoration={markings,mark=at position 0.4 with {\draw[myarrow](0,4pt)--(10pt,4pt);}},
    ] ([yshift=-1pt]myv-1-4.north west) -- (g.60);
\node[anchor=south west, inner sep=0pt] at ([xshift=2pt]bigsquare.south east) {$\mathcal{G}'_{f}$};
\end{tikzpicture}
\caption{The second graph}
\end{figure}
\end{document}