# How to nest a graph within a node in TikZ and draw edges between the graphs?

I'm trying to draw a graph and have a separate graph within one of the outer nodes. I seen this question, but I don't understand how the matrix nodes will help me. I would also like to draw edges between the outer nodes to the inner nodes.

Could someone please make a quick example of with 3 nodes, where one of the nodes contains a subgraph with 3 nodes?

Here's a little example: two "outer" nodes, each one containing a graph (formed with "inner" nodes) and some edges and arrows connecting outer nodes to outer nodes, and inner nodes from one graph to inner nodes of the other; the remember picture option lets you access inner and outer nodes at any time:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning,calc}

\begin{document}

\begin{tikzpicture}[remember picture,
inner/.style={circle,draw=blue!50,fill=blue!20,thick,inner sep=3pt},
outer/.style={draw=green,fill=green!20,thick,inner sep=10pt}
]
\node[outer,draw=green] (A) {
\begin{tikzpicture}
\node [inner,draw=blue] (ai)  {A1};
\node [inner,draw=blue,below=of ai] (aii) {A2};
\node [inner,draw=blue,right=of aii] (aiii) {A3};
\draw[red,thick] (ai) -- (aii) -- (aiii) -- (ai);
\end{tikzpicture}
};
\node[outer,draw=green,right=of A] (B) {
\begin{tikzpicture}
\node [inner,draw=blue] (bi)  {B1};
\node [inner,draw=blue,below=of bi] (bii) {B2};
\node [inner,draw=blue,right=of bii] (biii) {B3};
\node [inner,draw=blue,right=of bi] (biv) {B4};
\draw[red,thick] (bi) -- (bii) -- (biii) -- (biv) -- (bi) -- (biii);
\end{tikzpicture}
};
\draw[thick,orange,->] (ai) -- (bii);
\draw[orange,->] (aiii) -- (bi);
\draw[orange,->] (A.90) -- ($(A.90)+(0,1)$) -| (B);
\end{tikzpicture}

\end{document} I don't know the kind of problems that OP has with TiKZ matrices but Gonzalo's example is the kind of graph where a matrix is really helpful.

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning,matrix}

\begin{document}

\begin{tikzpicture}[
inner/.style={circle,draw=blue!50,fill=blue!20,thick,inner sep=3pt},
outer/.style={draw=green,fill=green!20,thick,inner sep=10pt, column sep=1cm, row sep=1cm}
]
\matrix (A) [matrix of nodes, outer, nodes={inner}]{
A1 \\
A2 & A3\\
};

\matrix (B) [matrix of nodes, outer, nodes={inner}, right=of A]{
B1 & B4\\
B2 & B3\\
};

\draw[red, thick] (A-1-1)--(A-2-1)--(A-2-2)--(A-1-1);
\draw[red, thick] (B-1-1)--(B-1-2)--(B-2-2)--(B-2-1)--(B-1-1);
\draw[thick, orange,->] (A-1-1)--(B-2-1);
\draw[thick, orange,->] (A-2-2)--(B-1-1);
\draw[thick, orange,->] (A.north)--([yshift=1cm]A.north)-|(B);
\end{tikzpicture}

\end{document} Building on Gonzalo Medina answer I figured out this neat code, that is useful when one wants to duplicate graph components. It also automates indexing to some degree.

\documentclass{standalone}
\usepackage{tikz}

\newcommand{\blocknode}{
\begin{scope}[every node/.style={thick, circle, draw ,minimum width=5mm}]
\draw (0,0) -- (0,5) -- (5,5) -- (5,0) -- cycle;
\node (T#1) at (2.5, 1){T#1};
\node (L#1) at (  1, 4){L#1};
\node (R#1) at (  4, 4){R#1};
\draw (L#1) to (T#1);
\draw (R#1) to (T#1);
\end{scope}
}

\begin{document}

\begin{tikzpicture}[remember picture]
\node (N1) at (0,0){\tikz{\blocknode{1}}};
\node (N2) at (9,3){\tikz{\blocknode{2}}};
\draw[color=red] (L1) -- (T1) --(R1) --(L1);
\draw[color=blue] (L2) -- (T2) --(R2) --(L2);
\draw[color=green] (L1) -- (L2) (R1)--(R2) (T1) --(T2);
\end{tikzpicture}

\end{document} 