Elements of Android Room Version 0.3 by Mark L. Murphy

Author:Mark L. Murphy
Language: eng
Format: mobi, epub
Publisher: CommonsWare
Published: 2020-09-20T15:11:58+00:00

(from MiscSamples/src/main/java/com/commonsware/room/misc/manytomany/BookCategoryJoin.kt)

The BookCategoryJoin class has our keys: isbn to point to a Book and shortCode to point to a Category. Hence, each BookCategoryJoin instance (or row in its table) represents one relationship between a Book and Category.

The @Entity annotation is more complex:

We use the primaryKeys property to say that the combination of isbn and shortCode is the primary key for our table

We set up indices on each of those columns, as we will be querying this table a lot to find all categories for a book or all books for a category

We have two @ForeignKey annotations, tying this class to Book and Category, and using onDelete = CASCADE to ensure that when we delete a Book or Category that its corresponding join entry gets deleted

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