Room Relation Specify Column Names When Extending - kotlin

Question

I am working with some datasets in my room database, and my method is to have one table with information about a dataset called DatasetInfo which stores things like name, type of value stored, id, etc; and a second table where I store the values in 3 columns: (id, date, value). This ordered triplet is defined as a DatasetValue entity. Here, (date, value) is an ordered pair that I want to plot.
To plot these ordered pairs, I have to convert them to a list of Entry objects, where Entry takes the values x and y. It makes the most sense to query my database and simply ask for List<Entry>, because right now I ask for List<DatasetValue> and then I have to map that result to List<Entry> which is unnecessary.
I query for the dataset information table DatasetInfo as follows:
data class DatasetWithValues(
#Embedded
var datasetInfo: DatasetInfo,
#Relation(
parentColumn = DATASET_COLUMN_DATASET_ID,
entityColumn = VALUES_COLUMN_ID,
entity = DatasetValue::class,
)
var values : List<Entry>
)
Now, as I said above, Entry has values x and y, and Dataset calls them date and value. Of course, when I ask for this relation, it will fail because it doesn't know how to assign values from a table with the columns id, date, and value to an object which takes x and y. So, I define a new class:
class DatasetEntry(
#ColumnInfo(name = "date")
var date : Float,
#ColumnInfo(name = "value")
val value : Float
) : Entry(date, value)
and then make the following adjustment:
//var values : List<Entry>
var values : List<DatasetEntry>
That does nothing. The code doesn't compile because:
SQL error or missing database (no such column: x)
Well, what if I instead write:
class DatasetEntry(
#ColumnInfo(name = "date")
var date : Float,
#ColumnInfo(name = "value")
val value : Float
) : Entry(){
init{
x = date
y = value
}
}
That doesn't help either, same error. Even if I remove that init call, it still wants x.
The plot thickens, because inside of Entry I can see x is declared private. So I have absolutely no clue what is happening here. How does Room even know to look for x? Is there any work around for this other than renaming the columns in my table to x and y?

Answer 1

Is there any work around for this other than renaming the columns in my table to x and y?
If you have such option it would be the easiest. Still there are some options you could consider:
1. Mapping Room's result to needed one
So, you ask Room for some raw result and then map it to ready one. For that you add 2 classes:
data class DatasetWithValuesRaw(
#Embedded
var datasetInfo: DatasetInfo,
#Relation(
parentColumn = DATASET_COLUMN_DATASET_ID,
entityColumn = VALUES_COLUMN_ID,
)
var values : List<DatasetValue>
)
data class DatasetWithValuesReady(
var datasetInfo: DatasetInfo,
var values : List<Entry>
)
Let's say you have a dao method:
Query("select * ....")
fun getRawData(): List<DatasetWithValuesRaw>
For mapping you use:
fun getReadyData() =
getRawData().map { item ->
DatasetWithValuesReady(item.datasetInfo,
item.values.map { Entry(x = it.date, y = it.value)
}) }
2. Replacing Room's #Relation with explicit query
It's not what you really want, but still is an option.
Use class like that:
data class DatasetWithSeparateValues(
#Embedded
var datasetInfo: DatasetInfo,
#Embedded
var value : Entry // <----- it's not a list, just a single value
)
and in your dao you set query with explicit columns' names (x and y). Something like that:
Query("SELECT *, values.date as x, values.value as y FROM dataset LEFT JOIN values on dataset.DATASET_COLUMN_DATASET_ID = values.VALUES_COLUMN_ID")
fun getData(): List<DatasetWithSeparateValues>
As a result you'll get a list, but if there is a one dataset with 5 values you'll get inside list 5 items with the same dataset and separate values. After that you could use Kotlin collection's methods (groupBy for example) to prettify result in some way