Kotlin Room SUM operation-CodePudding

I need to perform column sum operation But I don't understand what exactly I need to insert into fun getSumItem(): Flow<Cursor>

this is my Dao

@Dao
interface Dao {
    @Insert
    fun InsertItem(item: Income)
    @Query("SELECT SUM(sum) AS value FROM income")
    fun  getSumItem(): Flow<Cursor>

}

this is my table

@Entity(tableName = "income")
data class Income(
    @PrimaryKey(autoGenerate = true)
    var id: Int? = null,
    @ColumnInfo(name = "date")
    var date: String,
    @ColumnInfo(name = "type")
    var type: String,
    @ColumnInfo(name = "sum")
    var sum: Float,


)

I did not find a similar example and any detailed information on the Internet. when I use Cursor i get this error:
Entities and POJOs must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type). - android.database.Cursor

all other options that I found on the Internet also gave errors

CodePudding user response：

When you say SELECT SUM(sum) AS value FROM income the output that Room handles will be a Cursor with a single column named value. Room does not know how to extract this value and build a Cursor object from that single value.

Room expects to extract data from the underlying Cursor that is generated by the SQLite API, which Room is a wrapper to/around, and deliver the data according to the object that the function returns (and thus must be able to ascertain how to build the resultant object).

As Room extracts a single value then you simply need to retrieve the value returned as a Float.

fun  getSumItem(): Flow<Float>