Room entities and domain models, should they be separate?

Question

It seems to me that many guides on Room (and other ORMs for that matter) focus on the creation of Room entities and go on to use these as their domain models from then on. But what if my model needs its actual structure to perform some business logic?

For example, take the following class:

class Report(var id: Long, var patient: Patient, var surgery: Surgery) {

    var minimumAllowableBloodLoss: Double = 0.0
        get() = ((this.patient.hemoglobin - this.patient.minHemoglobin) / this.patient.hemoglobin) * this.patient.bloodVolume * this.patient.weight
        private set

    var hourlyDiuresis: Double = 0.0
         get() = this.patient.diuresisOutput / this.surgery.duration
         private set

    var urineOutput: Double = 0.0
        get() = this.hourlyDiuresis / this.patient.weight
        private set

    var intakeSupply: Double = 0.0
        get() = this.patient.totalIntake / this.patient.weight
        private set

    var finalFluidBalance: Double = 0.0
        get() = this.patient.totalIntake - this.patient.totalOutput
        private set

}

If I made this class into a Room entity, I would've had to change my object references to just foreign keys, essentially making impossible doing the calculations I need from this class.

Naturally, my first instinct was to scrap that idea entirely and create a representation object, I believe it's also called a "persistance model":

@Entity
data class ReportRow(
    var patientId: Long, var surgeryId: Long) {
    @PrimaryKey(autoGenerate = true)
    var id: Int = 0
}

But this also means I would have to create conversion methods from the persistence model to domain and vice versa.

This led me to believe maybe I'm missing something entirely or I'm just not using the tools correctly, is there a better alternative for these cases?

Answer 1

They should be separate.

From the docs:

Entity: Represents a table within the database.

Despite the documention referring often to entities as "data models", many examples simplify the design and assume entities = business models.

However, such design is likely to violate Single Responsibility Principle and quickly falls apart once you wish to keep your data in the cloud, where data may be kept using some no-SQL solution.