Default Values and Partial Entities

We have already seen how Room supports Kotlin default values on properties:

class CustomColumnNameEntity(
  @PrimaryKey
  val id: String,
  val title: String,
  @ColumnInfo(name = "words") val text: String? = null,
  val version: Int = 1
) {

Here, text and version both have default values. Really, these have nothing to do with Room — they are a pure Kotlin concept. However, in the end, if we create a CustomColumnNameEntity instance without supplying text or version, we get those defaults.

But, in Room 2.2.0, we got another option for default values… one that on the surface seems pointless, as it does not work with @Insert operations. Principally, this appears to be tied to another feature added in that same release: partial entity support.

In this chapter, we will explore both of these features.

Default Values, and the Other Default Values

The new option for default values comes in the form of a defaultValue property on the @ColumnInfo annotation:

@Entity(tableName = "defaultValue")
class DefaultValueEntity(
  @PrimaryKey
  val id: String,
  val title: String,
  @ColumnInfo(defaultValue = "something")
  val text: String? = null,
  @ColumnInfo(defaultValue = "123")
  val version: Int = 1
) {

Here, as in the example shown earlier, we have text and version properties on an entity (DefaultValueEntity), and in Kotlin, we have those properties default to null and 1, respectively. However, we also have @ColumnInfo annotations on those, where each has a defaultValue property. The one on text says the default value is something, while the one on version says that the default value is 123. So, for each property, we have two separate default values declared: one via Kotlin and one via the defaultValue annotation property… and both are different.

The Kotlin default values are for Kotlin’s use. You can create instances of DefaultValueEntity without supplying values for text and/or version.

The @ColumnInfo defaultValue properties are for SQLite’s use. If you attempt to INSERT a value into the table without providing values for the text and/or version columns, the defaultValue properties take effect. Basically, the defaultValue properties are embedded directly in the CREATE TABLE SQL statement generated from our @Entity declaration:

CREATE TABLE IF NOT EXISTS `defaultValue` (`id` TEXT NOT NULL, `title` TEXT NOT NULL,
  `text` TEXT DEFAULT 'something', `version` INTEGER NOT NULL DEFAULT 123,
  PRIMARY KEY(`id`))