Apple WebObjects 3.5 - Manual (Page 64)

Chapter 3 Creating a WebObjects Database Application
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MOVIE_ROLE table having a MOVIE_ID column. In the MOVIE table,
MOVIE_ID is a primary key, while in MOVIE_ROLE it’s a foreign key.
A primary key is a column or combination of columns whose values are
guaranteed to uniquely identify each row in that table. For example, each row
in the MOVIE table has a different value in the MOVIE_ID column, which
uniquely identifies that row. Two movies could have the same name but still be
distinguished from each other by their MOVIE_IDs.
A foreign key matches the value of a primary key in another table. The purpose
of a foreign key is to identify a relationship from a source table to a destination
table. In the following diagram, notice that the value in the MOVIE_ID column
for both MOVIE_ROLE rows is 501. This matches the value in the
MOVIE_ID column of the “Alien” MOVIE row. In other words, “Ripley”
and “Ash” are both roles in the movie “Alien.”
Suppose you fetch a Movie object. Enterprise Objects Framework takes the
value for the movie’s MOVIE_ID attribute and looks up movie roles with the
corresponding MOVIE_ID foreign key. The framework then assembles a
network of enterprise objects that connects a Movie object with its MovieRole
objects. As shown below, a Movie object has a vector of its MovieRoles, and the
MovieRoles each have a Movie.
1028
Ripley
501 501
Alien
1132
Ash
501 703
Toy Sto
ry
TALENT
_ID MOVIE_ROLE
MOVIE_ID MOVIE_ID
TITLE
MOVIE_ROLE
MOVIE