Develop Reference

Migration in Room with an Android Array of Strings

I have a database in Android with Room from which I have deleted a column. I was doing the migration, and I saw that it was not as simple as doing a DROP of the deleted column.
Then I have seen that I have to take a series of steps, creating a provisional table that will later be the new table with the deleted column, but the problem is that this table contains a field that is a String Array that I don't know how to declare in SQL.
#Entity(tableName = "recipe_table")
data class RecipesDb(
#PrimaryKey
#ColumnInfo(name = "id")
val id: Long,
#ColumnInfo(name = "name")
val name: String,
#ColumnInfo(name = "category")
val category: List<String>,
#ColumnInfo(name = "isRecommended")
val isRecommended: Boolean,
#ColumnInfo(name = "images")
val images: List<String>,
#ColumnInfo(name = "ingredients")
val ingredients: List<String>,
#ColumnInfo(name = "date")
val date: Long,
#ColumnInfo(name = "time")
val time: Int,
#ColumnInfo(name = "difficult")
val difficult: String,
#ColumnInfo(name = "originalUrl")
val originalURL: String? = null,
#ColumnInfo(name = "author")
val author: String,
#ColumnInfo(name = "siteName")
val siteName: String
)
And now I have removed the ingredients column. I wanted to do something like this:
private val MIGRATION_3_2 = object : Migration(3,2) {
override fun migrate(database: SupportSQLiteDatabase) {
//Drop column isn't supported by SQLite, so the data must manually be moved
with(database) {
execSQL("CREATE TABLE Users_Backup (id INTEGER, name TEXT, PRIMARY KEY (id))")
execSQL("INSERT INTO Users_Backup SELECT id, name FROM Users")
execSQL("DROP TABLE Users")
execSQL("ALTER TABLE Users_Backup RENAME to Users")
}
}
}
But when I declare the new temporary table User_Backup, I have no idea how to specify that one of the fields is an Array. In the end I was able to do it with Room's AutoMigrations and creating an interface, but I would like to know how to do it this way as well.

The simple way is to compile the code (Ctrl+F9) with the changed #Entity annotated classes in the list of entities of the #Database annotation.
Then look at the generated java (visible via the Android View in Android Studio). There will be a class that is the same name as the #Database annotated class but suffixed with _Impl.
In this class there will be a method that is named createAllTables, This includes the SQL that room uses for creating the tables.
Just copy and paste the appropriate SQL and then change the table name, this will not only use the correct type but also apply the correct column constraints that Room expects.
I would suggest
Adding an execSQL("DROP TABLE IF EXISTS the_backup_table_name;") before you create a table (just in case it already exists)
And instead of using execSQL("DROP TABLE Users") to use execSQL("DROP TABLE IF EXISTS the_original_table_name")
Personally I always RENAME the table name of the original, then RENAME the new table and then finally DROP the renamed original.
I would use:-
private val MIGRATION_3_2 = object : Migration(3,2) {
override fun migrate(database: SupportSQLiteDatabase) {
//Drop column isn't supported by SQLite, so the data must manually be moved
with(database) {
execSQL("DROP TABLE IF EXISTS Users_Backup")
execSQL("CREATE TABLE IF NOT EXISTS ....) //<<<<< SEE NOTES BELOW, the SQL MUST BE CHANGED.
execSQL("INSERT INTO Users_Backup SELECT id, name FROM Users")
execSQL("ALTER TABLE Users RENAME TO Old_Users")
execSQL("ALTER TABLE Users_Backup RENAME to Users")
execSQL("DROP TABLE IF EXISTS Old_users")
}
}
}
note .... indicates that the SQL is copied from the generated java and that the table name is changed from Users to Users_Backup
The first line will drop the Uers_backup just in case it happens to exist, it's just a little less likely to fail under unusual circumstances.
Rather than dropping the Users table before the RENAME of the Users_Backup to Users. The 4th execSQL changes the name of the Users table, so should there be an issue with changing the Users_Backup table to be the Users table, then the original Uers table is available as Old_users.
When all has been complted then the original Users table, now named Old_Users is then dropped.
These are all just a little safer/secure.

kotlin android Realm DB create a foreign key (Linking Objects)

I have 2 tables student and a teacher.
The student table has 3 fields, name, roll_no, subjects.
Roll_no being primary key.
Another table named marks with 4 fields subject ID, subject name, subject score and roll_no
roll_no field will be a foreign key.
How to create a relation like the marks table refers as a foreign key to student table's roll_no.

Short answer: Realm doesn't have foreign keys.
Long answer: Realm wants you to think about your data as objects, instead of tables and links (https://realm.io/docs/kotlin/latest/#relationships).
Assuming roll_no is kind of like a student_id - you could model your data like this:
open class Student(
#PrimaryKey
var id: String = "",
var markedSubjects: RealmList<MarkedSubject> = RealmList()
): RealmObject()
open class MarkedSubject(
var subject: Subject? = null,
var mark: Int? = null
): RealmObject()

How to Migrate Not Null table column into Null in Android Room database

I'm new to android room library. I need to migrate a Not Null column to Null,
But room migration only allow ADD or RENAME in ALTER table query. How do execute a column migration query?
#Entity(tableName = "vehicle_detail")
data class VehicleDetailsEntity(
#PrimaryKey(autoGenerate = true)
val vehicleClientId: Long = 0,
val vehicleId: String,
val updatedOn: Date,
val updatedBy: String
)
I need to change table structure into
#Entity(tableName = "vehicle_detail")
data class VehicleDetailsEntity(
#PrimaryKey(autoGenerate = true)
val vehicleClientId: Long = 0,
val vehicleId: String,
val updatedOn: Date?,
val updatedBy: String?
)
java.lang.IllegalStateException: Room cannot verify the data integrity. Looks like you've changed schema but forgot to update the version number. You can simply fix this by increasing the version number.

You need to run a migration since SQLite doesn't allow column constraint modification.
For that migration you need to create a new temp table and copy all your previous data to it, then delete the old table and rename the temp one to the needed table name.
If you have a scheme directory, you can find your exact creation SQL query which you should copy on your migration (I just figured it out from a scheme of mine and could not be 100% correct):
val MIGRATION_1_2: Migration = object : Migration(1, 2) {
override fun migrate(database: SupportSQLiteDatabase) {
// Create the new table
database.execSQL(
"CREATE TABLE IF NOT EXISTS VehicleDetailsEntityTmp (vehicleId TEXT NOT NULL, updatedOn TEXT, updatedBy TEXT,vehicleClientId INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL )"
)
// Copy the data
database.execSQL(
"INSERT INTO VehicleDetailsEntityTmp (vehicleId, updatedOn, updatedBy ,vehicleClientId) SELECT vehicleId, updatedOn, updatedBy ,vehicleClientId FROM VehicleDetailsEntity ")
// Remove the old table
database.execSQL("DROP TABLE VehicleDetailsEntity")
// Change the table name to the correct one
database.execSQL("ALTER TABLE VehicleDetailsEntityTmp RENAME TO VehicleDetailsEntity")
}
}

Primary keys in Room should be Int or Long?

I'm designing a Database that would be implemented in Android using Room, after reading the docs I found that there is no recomendations about using Int or Long as primary keys.
In some places they define entities with int primary keys:
#Entity
data class User(
#PrimaryKey var id: Int,
var firstName: String?,
var lastName: String?
)
But in other place it says that if you want to get the ID of the last row inserted "insert" method return a long.
#Dao
interface MyDao {
#Insert(onConflict = OnConflictStrategy.REPLACE)
fun insertUsers(vararg users: User)
#Insert
fun insertBothUsers(user1: User, user2: User)
#Insert
fun insertUsersAndFriends(user: User, friends: List<User>)
}
If the #Insert method receives only 1 parameter, it can return a long, which is the new rowId for the inserted item. If the parameter is an array or a collection, it should return long[] or List instead.
So, The primary keys in room should be Int or Long?; Are there best practices about choosing one type over the other?

Both of these types will map to an INTEGER in the underlying SQLite database.
For example, with a class like this:
#Entity
data class Test(#PrimaryKey val i: Int, val l: Long)
You'd get a SQLite table defined with this query:
CREATE TABLE IF NOT EXISTS `Test` (`i` INTEGER NOT NULL, `l` INTEGER NOT NULL, PRIMARY KEY(`i`))
So you can use whichever one you'll need the magnitude of in your code. If you do decide to use an Int for some reason and you run out of values, you can even change it to a Long without having to migrate your database later on.
As for this INTEGER type, as per the SQLite documentation:
The value is a signed integer, stored in 1, 2, 3, 4, 6, or 8 bytes depending on the magnitude of the value.

Both is fine. On mobile (and most of the time in general) Int should be sufficient (it will also save you 4 bytes over Long).
Why? Using an Int you could store over 2 billion records (2_000_000_000). So you could store a record of around 1/4 of all the humans living on earth. Just for comparison: Using a Long would enable you to store over 900 quadrillion records (900_000_000_000_000_000).

I would recommend to use Long since the insert function will return a Long, or long[].

Android Room Migration: Update Attribute Name By Creating New Table

Issue
In my project I a have a room table named 'content' with a Double attribute 'archivedCount'. In the latest version of the app the attribute archivedCount attribute is re-named to dismissCount, still as type Double.
Android API Level / SQL Version
28 / 3.19
Original Content model
#Entity(tableName = "content")
data class Content(#PrimaryKey var id: String, var archiveCount: Double) : Parcelable {...}
New Content model
#Entity(tableName = "content")
data class Content(#PrimaryKey var id: String, var dismissCount: Double) : Parcelable {...}
Runtime error
java.lang.IllegalStateException: Migration didn't properly handle content(app.coinverse.content.models.Content).
I've inspected the Expected and Found tables the log prints and they appear to be identical.
Attempted Solution
I attempted the complex schema change as outlined by a Google Developer Advocate unsuccessfully in order to modify the name of one attribute / column. Here is a basic version of what I attempted. 
val MIGRATION_1_2: Migration = object : Migration(1, 2) {
override fun migrate(database: SupportSQLiteDatabase) {
// Create the new table
database.execSQL("CREATE TABLE content_new (id TEXT, dismissCount REAL, PRIMARY KEY(id))")
// Copy the data
database.execSQL("INSERT INTO content_new (id, dismissCount) SELECT id, archiveCount FROM content")
// Remove the old table
database.execSQL("DROP TABLE content")
// Change the table name to the correct one
database.execSQL("ALTER TABLE content_new RENAME TO content")
}
}

Can't see anything wrong with your implementation, I would suggest that you use a different #Entity class that is not named Content() and try again.