IssueTracker

Component used: Room
Version used: 1.0
Devices/Android versions reproduced on: NA

I have one entity that contains a set of enums. For example:

@Entity
data class Person(
        @field:PrimaryKey val id: Int,
        val name: String,
        val favouriteDays: Set<Day>)

enum class Day {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
}

I want to store this entity in a single table. In many cases, especially with enums, we could use a single bit mask to save the list as a number in the database.

Here's one example of what I wanted to do:

object DayCollectionTypeConverter {
    @JvmStatic
    @TypeConverter
    fun daysConverter(days: Set<Day>): Int {
        var value = 0
        for (day in days) {
            value += (1 shl day.ordinal)
        }
        return value
    }

    @JvmStatic
    @TypeConverter
    fun daysConverter(value: Int): Set<Day> {
        val set = mutableSetOf<Day>()
        for (day in Day.values()) {
            if (value and 1 shl day.ordinal != 0) {
                set.add(day)
            }
        }
        return set
    }
}

That code doesn't work as intended because @TypeConverter would allow me to convert one Day object, but not a collection (or set, list, etc) of Days.

I suggest a new annotation is created so the developer can specify how to save a Collection into a single column in the same table.