Apple WebObjects 3.5 Instrukcja Użytkownika Pobierz pdf (Strona 64)

Chapter 3 Creating a WebObjects Database Application

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.

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Apple WebObjects 3.5 Instrukcja Użytkownika Strona 64