CODEWITHSUNDEEP
androidsqltype-conversionandroid-room
Mr.Drew
Mr.Drew

Reputation: 1089

Android Room - How to migrate when a nested object model is changed?

I have a database schema set up using Android Room, Dao, and Entity classes set up as POJOs. Except the POJO entity isn't so "plain" and that it actually holds a reference to another object. I thought this was a great idea at the time as it allowed me more flexibility in changing the object and using it in other places in the app and only saving to the database as needed.

The problem I'm facing now is that the migration guideline only mentions how to migrate the database by altering the SQL, but I changed the object itself. My typeconverter class simply converts the object to and from a string.

Because it's being saved as a long string I know I essentially have to do a simple REPLACE(string, old_string, new_string) in the SQL migration code block with the updated object being the new string. How can I retrieve the old objects and update values before running the replace SQL command in the migration block?

UPDATE: I'm using GSON in my typeconverter class to change the object to a string, so the solution that comes to mind is to simply download the old object and upload the new one with the added fields. Only problem is that you can't access the database and download the json, convert it to the object, add the new data fields, then reconvert to a new json string.

I'm lucky I'm not at scale yet because this would be a tricky thing to do for so many users. (So I recommend that anyone reading this not do what I did and implement object nesting. It's easier to convert the Entry objects to the other portable objects instead of nesting when it comes to updating the data you want saved.)

I think if you already did what I did and can't go back, the best bet is to simply create the new portable object and make new typeconverter functions for that one and add the SQL COLUMN for the new object. The problem then lies in how you then retrieve those objects from the Entry Dao, which will cause a lot more code to write and possible errors to debug if not done carefully.

Long story short, if anyone is reading this, DO NOT nest objects in Room DBs on Android unless you are 100% sure it's a final form of your model... but is there such a thing anyways?

Upvotes: 4

Views: 1389

Answers (1)

lasec0203
lasec0203

Reputation: 2663

I just ran into this issue, but fortunately I only needed to add a new key/value pair to a "flat" object model. So hopefully my answer can be expanded on to fully answer @Mr.Drew question.

Assuming you have a table town with a column star_citizen that is the object model being typeconverted:

{"name":"John", "age":30, "car":false}

and you want to update the object to have an extra property "house": true

you could add a migration to your App's Room Database class like this (Kotlin example):

@Database(entities = [Town::class], version = 2,  exportSchema = true)
@TypeConverters(DataConverters::class)
abstract class AppDatabase : RoomDatabase() {
    abstract val sharedDao : SharedDao

    companion object {
        private val MIGRATION_1_2 = object : Migration(1, 2) {
            override fun migrate(database: SupportSQLiteDatabase) {
                val cursor = database.query("SELECT * FROM `town`")

                // iterate through each row in `town`, and update the json
                // of the StarCitizen object model
                cursor.moveToFirst()
                while (!cursor.isAfterLast) {
                    val colIdIdx = cursor.getColumnIndex("id")
                    val id = cursor.getInt(colIdIdx)

                    val colStarCitizenIdx = cursor.getColumnIndex("star_citizen")
                    val rawJson = cursor.getString(colStarCitizenIdx)

                    val updatedRawJson =  starCitizenModelV1ToV2(rawJson)

                    database.execSQL("""UPDATE town SET star_citizen ='${updatedRawJson}' WHERE ID = $id""")
                    cursor.moveToNext()
                }
            }
        }


        //[...]

        private fun starCitizenModelV1ToV2(rawJson: String): String {
            val rawJsonOpenEnded = rawJson.dropLast(1)
            val newProperty = "\"house\":true"

            return "$rawJsonOpenEnded,$newProperty}"
        }
    }
}

Upvotes: 2

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