Room as a Cache — Programuoki

Code

The Room cache — Entity, DAO, Database

Three files in the code column as tabs, in dependency order: MenuItemEntity.kt → MenuDao.kt → PicaDatabase.kt (a row, then how you read/write it, then the database that holds it).

1. MenuItemEntity.kt — the cache row. It's separate from the domain MenuItem (lesson 2): the entity lives in core/data/local (the storage layer), carries @Entity and @PrimaryKey, while MenuItem stays the clean model the UI renders. We map between them.

First add the Room dependencies in build.gradle.kts (stack-when-needed; Room 2.x, not 3.0):

implementation(libs.androidx.room.runtime)
implementation(libs.androidx.room.ktx)      // Flow + suspend support
ksp(libs.androidx.room.compiler)            // plugin: id("androidx.room") + id("com.google.devtools.ksp")

2. MenuDao.kt — how you read and write. The key asymmetry: reads return a Flow (a live stream — the UI re-renders when the table changes), writes are suspend (insertAll, clearAll).

3. PicaDatabase.kt — the database itself: @Database(entities = [MenuItemEntity::class], version = 1), exposing menuDao().

Gotcha (schema migration). version = 1 matters. If you later change the entity (add a column) and don't bump the version, Room throws an IllegalStateException about a schema mismatch. And if you bump the version (version = 2) without a migration you get:
java.lang.IllegalStateException: A migration from 1 to 2 was required but not found.
Fixes: write a Migration(1, 2) (preserves data), or — for a cache — .fallbackToDestructiveMigration() (drops and rebuilds; fine for a cache, since the server is the source of truth). Never just bump the version "to make it compile."

MenuItemEntity.kt CREATE

Create this file in app/src/main/java/eif/viko/lt/pica/core/data/local/

package eif.viko.lt.pica.core.data.local

import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity(tableName = "menu_items")
data class MenuItemEntity(
    @PrimaryKey val id: Int,
    val name: String,
    val description: String,
    val price: Double,
    val category: String,
    val imageUrl: String?
)

MenuDao.kt CREATE

Create this file in app/src/main/java/eif/viko/lt/pica/core/data/local/

package eif.viko.lt.pica.core.data.local

import androidx.room.Dao
import androidx.room.Insert
import androidx.room.OnConflictStrategy
import androidx.room.Query
import kotlinx.coroutines.flow.Flow

@Dao
interface MenuDao {
    @Query("SELECT * FROM menu_items")
    fun getMenu(): Flow<List<MenuItemEntity>>   // reads = a Flow (live stream)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertAll(items: List<MenuItemEntity>)   // writes = suspend

    @Query("DELETE FROM menu_items")
    suspend fun clearAll()
}

PicaDatabase.kt CREATE

Create this file in app/src/main/java/eif/viko/lt/pica/core/data/local/

package eif.viko.lt.pica.core.data.local

import androidx.room.Database
import androidx.room.RoomDatabase

@Database(entities = [MenuItemEntity::class], version = 1)
abstract class PicaDatabase : RoomDatabase() {
    abstract fun menuDao(): MenuDao
}