\begin{frame}
  \frametitle{Aggregation}

  Consider the \textit{works-on} relation we have seen before.
  \mpause[1]{
  \begin{goal}{}
  We now want to express that a task performed by an employee
  might have a manager assigned to it.
  \end{goal}
  }
  \mpause{
  \begin{itemize}
    \item ER model has \alert{no relations between relations}
  \end{itemize}
  }
  \begin{exampleblock}{}
    \begin{center}\vspace{-4ex}
      \scalebox{.9}{
      \begin{tikzpicture}[every edge/.style={link}]
        \node[entity] (employee) {employee};
        \node[entity,right of=employee, node distance=6cm] (branch) {branch};
        \node[entity,right of=employee, node distance=3cm,yshift=2cm] (job) {job};

        \node[relationship,scale=.9,aspect=1.3] (works-on) at ($(employee)!.5!(branch)$) {works-on} edge (employee) edge (branch) edge (job);

        \mpause[1]{
        \node[entity,above of=employee, node distance=2cm,xshift=-2.5cm] (manager) {manager};
        \node[relationship,scale=.9,aspect=1.3] (manages) at ($(manager)+(2.5cm,0cm)$) {manages};
        \draw (manages) to (manager);
        \begin{scope}[thick]
        \mpause[3]{
        \draw (manages) to (employee);
        \draw (manages) to[out=45,in=90,looseness=1] (branch);
        \draw (manages) to (job);
        }
        \end{scope}
        }
      \end{tikzpicture}}
    \end{center}
  \end{exampleblock}
\end{frame}