\begin{frame}
  \frametitle{UML: Aggregation and Composition}

  \begin{goal}{Aggregation in UML}
    \begin{center}
    \begin{tikzpicture}
    \umlclass{system}{ 
      +name \\
      +location
      }{ 
      } 
    \umlclass[x=7]{component}{ 
      +type \\
      +model \\
      + serialNumber
      }{ 
      } 
    \umluniaggreg[arg2=$1..*$,mult2=$\,$,pos2=0.9]{system}{component} 
    \end{tikzpicture} \vspace{-2ex}
    \end{center}
    \begin{itemize}
      \item \emph{Aggregation:} system is a collection of components
    \medskip
      \item \emph{Composition:} if the diamond would be filled black,
            it would mean that every component belongs to one system ($1..1$)
    \end{itemize}
  \end{goal}
  \pause
    
  \begin{alertblock}{}
    It is important to note the difference with ER models:
    \begin{itemize}
      \item In ER aggregation allows to treat relations as entities.
      \item Composition in UML is similar to weak entities in ER.\\
            \remark{However, composition in UML says nothing about keys.}
    \end{itemize}
  \end{alertblock}
\end{frame}