How can I store currency in ROOM DB?

Question

I have entity:

@Entity(tableName = "products")
public class Product{

    @PrimaryKey(autoGenerate = true)
    private Long id;

    @ColumnInfo(name = "amount")
    private BigDecimal amount;

I need store it in Room DB. I Can not store BigDecimal and I create converter:

public static class Converters {
        @TypeConverter
        public BigDecimal fromString(String value) {
            return value == null ? null : new BigDecimal(value);
        }

        @TypeConverter
        public Double amountToString(BigDecimal bigDecimal) {
            if (bigDecimal == null) {
                return null;
            } else {
                return bigDecimal.doubleValue();
            }
        }
    }

and add to column:

@TypeConverters(Converters.class)
    @ColumnInfo(name = "amount")
    private BigDecimal amount;

Now I store currency in double column. And I want method for summ totalcurrency:

@Query("SELECT SUM(amount) FROM products")
LiveData<Double> totalSum();

But I think it is bad way, because I can lose some values when converting.

My question: How can I ctore currency in ROOM DB? and return method

LiveData<BigDecimal> totalSum();

Answer 1

Instead of Long vs BigDecimal converters, I'm using between String and BigDecimal.

Warning: With this way, you can't use SUM, TOTAL or other expression with Room DB. Because my purpose for app is using many currencies, so I manually retrieve data and calculate based on current exchange rates.

class BigDecimalTypeConverter {

    @TypeConverter
    fun bigDecimalToString(input: BigDecimal?): String {
        return input?.toPlainString() ?: ""
    }

    @TypeConverter
    fun stringToBigDecimal(input: String?): BigDecimal {
        if (input.isNullOrBlank()) return BigDecimal.valueOf(0.0)
        return input.toBigDecimalOrNull() ?: BigDecimal.valueOf(0.0)
    }

}

if you only have one currency, you can use Double instead of String:

class BigDecimalDoubleTypeConverter {

    @TypeConverter
    fun bigDecimalToDoubleinput: BigDecimal?): Double {
        return input?.toDoubleOrNull() ?: 0.0
    }

    @TypeConverter
    fun stringToBigDecimal(input: Double?): BigDecimal {
        if (input == null) return BigDecimal.ZERO
        return BigDecimal.valueOf(input) ?: BigDecimal.ZERO
    }

}

So you can retrieve SUM, TOTAL,... from Dao:

@Query("SELECT SUM(transactions.amounts) FROM transactions WHERE transactions.timestamp >= :fromDateTimestamp")
fun getTotalAmount(fromDateTimestamp: Long): Double

Hope it help!