Trigger on select :-(

What I want is to have a trigger on select, but since I have searched the
archives I know that is not possible.  I also read that view rules is the
technology is whats available.  Having never used views before I can't seem
to get my mind to figure out what I need to do to solve my problem.

I have a table that defines a key that I will use in another table in a list
of items defined in the previous table.

The tables are

CREATE TABLE Foo ( INTEGER key not null primary key,
                                  ...  characteristics for the Foo item );

CREATE TABLE Catalog ( INTEGER FooKey FOREIGN KEY,
                                        ... other catalog data );

The catalog will have rows that reference the elements in the Foo table.

I also have another table that references the Foo table

CREATE TABLE ToDo ( INTEGER FooKey FOREIGN KEY,
                                    ... other ToDo information.

What I want to do is when I do a SELECT on the Catalog and deliver the
result to the user, I want to check to see if the FooKey is in the users
ToDo table and set the value of a column isToDo to true or false depending
on if the FooKey exists in the ToDo table for the user.  If I were building
a table I would use a trigger and select on the row in the ToDo table.

Please, all I want is an idea where to start, what I should read and I will
figure out how to do it.

Thanks in advance.