How to Set Default Values in Room Database
When you initialize your application, you often need a default value to determine how it works. For example, most values of Settings require a default value. When you use a database, you read a value from there. Thus, it is better to put a default value in there instead of in the code.
How can you do it? Room database supports a callback method. There are two callbacks:
onCreate is called only when the database is created for the first time. After that, this is not called again.
onOpen is called when the database is opened. Both callbacks are not called when your application is started but called when your code accesses the database such as querying.
If you want to pre-populate data only one time, you can do it using
onCreate callback. However, it wouldn’t fit well if there is a chance to add another default value later. If you think about Settings, there could be a new setting option in the next version.
When the application is restarted,
onOpen can be called unlike
onCreate. Now the remaining question is how to set default values properly. Since
onOpen can be called several times, you need to pre-populate data only if it is necessary. If not, you could end up overriding data changed by a user.
This sounds like a bit tricky but actually quite simple to do. Room database supports to select a conflict algorithm. By selecting,
CONFLICT_IGNORE, you can set default values without worrying overriding. Room database doesn’t allow to access the database on the main thread unless you set
allowMainThreadQueries(). Thus, the callback method is executed not on the main thread.