How can I make xtable work with aggregate(..., FUN = paste)?

Question

Say I have the following data frame:

df <- data.frame(sector = c(rep('a', 3), rep('b', 4)), id = 1:7)

I would like to create a summarized table reporting a list of IDs on each sector, so I do this:

table <- xtable(aggregate(id ~ sector, df, paste))

However, when I try to use xtable() to transform the object above into LaTeX, I get the following error:

> xtable(table)
Error in is.finite(x) : default method not implemented for type 'list'

Which is caused by the second column, since class(table$id) confirms it is a list.

It should be noted that xtable() handles data frames as input well. Changing "paste" to something more common like "sum" or "length" doesn't yield errors.

What can I do to table (or to xtable) so it creates the table I want? The desired raw output should look something like this:

\begin{table}[ht]
\centering
\begin{tabular}{rlr}
  \hline
 & sector & id \\ 
  \hline
1 & a &   1, 2, 3 \\ 
  2 & b &  4, 5, 6, 7\\ 
   \hline
\end{tabular}
\end{table}

Answer 1

Adding a 'collapse' argument to paste will succeed:

> xtable(aggregate(id ~ sector, df, paste, collapse=","))
% latex table generated in R 3.0.1 by xtable 1.7-1 package
% Tue Aug  6 11:46:03 2013
\begin{table}[ht]
\centering
\begin{tabular}{rll}
  \hline
 & sector & id \\ 
  \hline
1 & a & 1,2,3 \\ 
  2 & b & 4,5,6,7 \\ 
   \hline
\end{tabular}
\end{table}