\documentclass{article}
\usepackage{tikz,amsmath}
%\usepackage{siunitx}
\usetikzlibrary{arrows,snakes,backgrounds,patterns,matrix,shapes,fit,calc,shadows,plotmarks}
\usepackage[graphics,tightpage,active]{preview}
\PreviewEnvironment{tikzpicture}
\PreviewEnvironment{equation}
\PreviewEnvironment{equation*}
\newlength{\imagewidth}
\newlength{\imagescale}
\pagestyle{empty}
\thispagestyle{empty}

\usepackage{standalone}

\begin{document}

\begin{tikzpicture}
\tikzstyle{main}=[circle, minimum size = 10mm, thick, draw =black!80, node distance = 16mm]
\tikzstyle{hyparam}=[rectangle, minimum size = 5mm, thick, draw =black!80, fill = black!10, node distance = 16mm]
\tikzstyle{connect}=[-latex, thick]
\tikzstyle{selector}=[-latex, -|, snake=snake,segment amplitude=.4mm,segment length=2mm,line after snake=1mm, thick]
\tikzstyle{shortconnect}=[-latex, thin]
\tikzstyle{box}=[rectangle, draw=black!100]
\tikzstyle{switch}=[circle, minimum size = 1mm, fill = black!100, draw=black!100]
  \node[hyparam] (alpha0) [label=below:$\alpha_0$] { };
  \node[main] (pi) [right of=alpha0,label=below:$\boldsymbol\pi$] {[K]};
  \node[main] (z) [right of=pi,label=below:$z_i$] {K};
  \node[main] (mu) [above of=pi,yshift=10mm, label=below:$\boldsymbol\mu_k$] {[D]};
  \node[main, fill = black!10] (x) [right of= z,label=below:$\mathbf{x}_i$] {[D]};
  \node[main] (Lambda) [above of=z,yshift=10mm, label=below:$\boldsymbol\Lambda_k$] {[D,D]};
  \node[hyparam] (nu0) [above of=Lambda, label=$\nu_0$] { };
  \node[hyparam] (W0) [right of=nu0, label=$\mathbf{W}_0$] {[D,D]};
  \node[hyparam] (beta0) [above of=mu, label=$\beta_0$] { };
  \node[hyparam] (mu0) [left of=beta0, label=$\boldsymbol\mu_0$] {[D]};
  %\node[switch] (xz) [above left of=x] {};
  \path (alpha0) edge [connect] (pi)
        (pi) edge [connect] (z)
        %(z) edge [selector] (xz)
        %(mu) edge [connect] (xz)
        %(Lambda) edge [connect] (xz)
        %(xz) edge [shortconnect] (x)
        (z) edge [selector] (x)
        (mu) edge [connect] (x)
        (Lambda) edge [connect] (x)
        (Lambda) edge [connect] (mu)
        (nu0) edge [connect] (Lambda)
        (W0) edge [connect] (Lambda)
        (beta0) edge [connect] (mu)
        (mu0) edge [connect] (mu);
  %\node[rectangle, inner sep=0mm, fit= (z) (x) (xz),label=below right:N, yshift=-1.5mm, xshift=7mm] {};
  \node[rectangle, inner sep=0mm, fit= (z) (x),label=below right:N, yshift=-1.5mm, xshift=7mm] {};
  \node[rectangle, inner sep=4.4mm,draw=black!100, fit= (z) (x), yshift=-1.5mm, xshift=0.5mm] {};
  \node[rectangle, inner sep=0mm, fit= (mu) (Lambda),label=below right:K, yshift=-2mm, xshift=11mm] {};
  \node[rectangle, inner sep=4.4mm,draw=black!100, fit= (mu) (Lambda), yshift=-2mm, xshift=3mm] {};
\end{tikzpicture}

\end{document}