Entity⌗

Using Room, tables are created with via an Entity. A class tagged @Entity then uses each property to represent a column in the table

@Entity(tableName = "word_table")
data class Word (@PrimaryKey @ColumnInfo(name = "word") val word: String)
  • When you want the name of the table to be different than the name of the class, use the tableName option of @Entity
  • Every entity needs a primary key, notated with @PrimaryKey
  • Similar to tableName, if you want the column name to not match the property name, use the name option of @ColumnInfo
  • You cannot have private properties in an entity

DAO⌗

DAO stands for ‘data access object’, and allows us to match up SQL queries to method calls. It must be an interface or abstract class. The included annotations for within a DAO include:

  • @Query, which requires a string query as a parameter
    @Query("SELECT * FROM word_table ORDER BY word ASC")
fun getAlphabetizedWords(): List<Word>
  • @Insert, which does not need a query
    • specify what to do about conflicts using the onConflict parameter. OnConflictStrategy.IGNORE tells the table to ignore an insert if the entry already exists
    @Insert(onConflict = OnConflictStrategy.IGNORE)
suspend fun insert(word: Word)

Sidenote: suspend tells the system that this function may hang while it waits for a response, and is related to coroutines

  • @Delete
  • @Update

Room⌗

The actual class that represents the database. Annotated with the @Database tag. Must be abstract.

// Annotates class to be a Room Database with a table (entity) of the Word class
@Database(entities = arrayOf(Word::class), version = 1, exportSchema = false)
abstract class WordRoomDatabase : RoomDatabase(){

    abstract fun wordDao(): WordDao

    companion object {
        // Singleton prevents multiple instances of database opening
        // at the same time.
        @Volatile
        private var INSTANCE: WordRoomDatabase? = null
        fun getDatabase(context: Context): WordRoomDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    WordRoomDatabase::class.java,
                    "word_database"
                ).build()
                INSTANCE = instance
                instance
            }
        }
    }
}