Saving Data on Android

answer.kt
 file under the 
data 
▸
 model
 package.
The 
Answer
 entity already has a 
question_id
 field that you can use as a foreign key 
that points to the 
question_id
 field of your 
Question
 entity. The problem is that 
you still have not told Room that this is actually a foreign key. For this, you need to 
use the 
foreignKeys
 property of the 
@Entity
 annotation to define the 
relationships.
Saving Data on Android
Chapter 7: Mastering Relations
raywenderlich.com
125

---

Add a 
foreignKeys
 property to the 
@Entity
 annotation of your 
Answer
 class like 
this:
@Entity(tableName = "answer", 
    foreignKeys = [//1 
        ForeignKey(entity = Question::class,//2 
            parentColumns = ["question_id"],//3 
            childColumns = ["question_id"],//4 
            onDelete = CASCADE)
//5
 
    ])
Taking each commented section in turn:
1.
foreignKeys
 allows you to define a relationship between this and another entity. 
This property accepts an array of 
ForeignKey
 objects that you can use to define 
foreign key constraints.
2. The first parameter in the constructor of a 
ForeignKey
 object accepts the entity 
to which this entity is related. In this case, you are passing the 
Question
 class 
since you want to create a foreign key constraint to the 
Question
 entity.
3.
parentColumns
 accepts the column names in the parent entity as an array. Since 
you want to match each answer to a single question, you are going to pass the 
primary key of your Question entity: 
question_id
.
4.
childColumns
 accepts the column names in the current entity to use as foreign 
keys.
5.
onDelete
 tells Room what to do in case the parent entity is deleted from the 
database. For example, what would happen to your answers if the respective 
question is deleted?  When you enter 
CASCADE
 for the value of 
onDelete
, be sure 
to use the import for 
androidx.room.ForeignKey.CASCADE
.
There are several options for 
onDelete
:
1.
CASCADE
: If a record is deleted from the parent entity each row in the child 
entity that was associated with the parent entity is also deleted. For this app, you 
are using this option since you want to delete the answers if the question is 
deleted.
2.