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.