Pro Android with Kotlin

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CHAPTER 8: APIs
Here are a few notes about this construct:

In Java you would write @Entity(foreignKeys = @ForeignKey( ... ) . 
Kotlin doesn’t allow annotations inside annotations. In this case, using 
the constructor serves as a substitute, which boils down to omitting the 
@ for inner annotations.

In a Java annotation, attribute value arrays are written like name = { 
..., ... }. This cannot be used in Kotlin because the curly braces do 
not serve as array initializers. Instead, the arrayOf(...) library method 
gets used.

The childColumns attribute points to the reference key in this entity, 
Contact.employeeId in this case.

The parentColumns attribute points to the referred-to foreign key entity, 
in this case Employee.uid.

The onDelete attribute tells what to do if the parent gets deleted. A value 
of ForeignKey.CASCADE means to also automatically remove all children, 
which is the associated Contact entities. The possible values are as 
follows:

CASCADE: Transport all actions to the root of the child-parent relation 
tree.

NO_ACTION: Don’t do anything. This is the default, and it leads to an 
exception if the relationship breaks because of update or delete 
actions.

RESTRICT: Similar to NO_ACTION, but the check will be made 
immediately when a delete or an update happens.

SET_NULL: All child key columns get set to null if a parent delete or 
update happens.

SET_DEFAULT: All child key columns get set to their default if a parent 
delete or update happens.

The onUpdate attribute tells what to do if the parent gets updated. 
A value ForeignKey.CASCADE means to also automatically update all 
children, which are the associated Contact entities. The possible values 
are the same as for onDelete.

The deferred = true setting will postpone the consistency check 
until the database transaction is committed. This might, for example, 
be important if both parent and child get created inside the same 
transaction.

Foreign keys must be part of a corresponding index. Here Contact.
employeeId gets the index. You’ll learn more about indexes in the 
following sections.

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