Default values for new properties

[werelord]

Im using ObjectBox in a pet android project written in Kotlin.. I have a data class encoded as such (example):

@Entity data class Foo(var name:String, var description: String) {
    constructor() : this("", "") // parameter-less constructor
    @Id var id: Long = 0L
    // other stuff
}

Database already existed on the device.. I added an additional property to the Foo class (not in the data constructor):

@Entity data class Foo(var name:String, var description: String) {
    constructor() : this("", "") // parameter-less constructor
    @Id var id: Long = 0L
    var bar: String = ""
    // other stuff
}

defaulted, non-null object bar.. however when pulling existing records from the stored box, that value bar appears to be null.. I would expect it to be a blank string (default for the parameter); is the violation of not-null guarantee coming from the Java interop?

I fixed the issue by nuking the database; afterwords the entity created and stored is not-null.. But this brings up any possible future updates that may violate this.

Is there a "database upgrade" method for updating already stored objects between versions, adding default values to new properties, or am I resigned to make anything added to an existing model be nullable (and handle it as such)?

[greenrobot]

It's quite easy to trick the Kotlin non-nullable types, e.g. with JSON parsers. Same is true for ObjectBox. We will target those kind of oddities in a future version. For now, we suggest to either use nullable types, or have some update code in place. Recipe for the latter:

1. Have a flag (via SharedPrefs for example) like "fooBarChecked"
2. If flag is false, run a migration and set the flag
3. Migration: run a query to get all Foos with bar == null and update the resulting list

[greenrobot]

Maybe an annotation for defining "defaults" for null values? E.g.

@DefaultForNull("")
var bar: String = ""

[greenrobot-team]

No response. Closing. -ut

[greenrobot-team]

Reopening to discuss default values (in Java, but also Kotlin classes with default values).

This also affects queries (merged from #589), e.g. if adding intProperty, then querying box with existing entities:

box.query().equal(intProperty, 0) // no results
box.query().isNull(intProperty) // has results

-ut

[greenrobot-team]

With 2.6.0-RC there is very initial support for a @DefaultValue annotation. Currently only String properties are supported, and only the empty string "" as default value.

Usage:

@DefaultValue("")
String exampleProperty;

@DefaultValue("")
var exampleProperty: String = ""

Edit: This annotation currently simply adds the new built-in NullToEmptyStringConverter to the property:

objectbox-java/objectbox-java/src/main/java/io/objectbox/converter/NullToEmptyStringConverter.java

Lines 8 to 22 in 220fc35

	public class NullToEmptyStringConverter implements PropertyConverter<String, String> {
	@Override
	public String convertToDatabaseValue(String entityProperty) {
	return entityProperty;
	}
	@Override
	public String convertToEntityProperty(@Nullable String databaseValue) {
	if (databaseValue == null) {
	return "";
	}
	return databaseValue;
	}
	}

So to customize default values, create your own converter and replace @DefaultValue with @Convert.

[greenrobot]

My first assessment is that we should keep default values separate from queries. E.g. if you want to, you can query for null values.