The following built-in types, their aliases and named types based on them are recognized as ObjectBox and stored as an appropriate internal type:

int, int8, int16, int32, int64
uint, uint8, uint16, uint32, uint64
bool
string, []string
byte, []byte
rune
float32, float64

Defining a converter

To add support for a custom type, you can map properties to one of the built-in types using a converter annotation.

For example, you could define a color in your entity using a custom Color struct and map it to an int32. Or you can map the time.Time to an int64, though losing some precision - less than a millisecond, i. e. a thousandth of a second):

type Task struct {
	Id          uint64
	Text        string
	DateCreated time.Time  `objectbox:"date type:int64 converter:timeInt64"`
}

In the entity definition above, we instruct ObjectBox to store the DateCreated field as a int64 while converting it to/from time.Time when using in the program. ObjectBox will generate a binding code that will call the following two functions (both start with the prefix timeInt64 specified above):

// from DB value to runtime value
func timeInt64ToEntityProperty(dbValue int64) (time.Time, error)
​
// from runtime value to DB value
func timeInt64ToDatabaseValue(goValue time.Time) (int64, error)

Just to complete the example, those functions could be implemented like this:

// converts Unix timestamp in milliseconds (ObjectBox date field format) to time.Time
func timeInt64ToEntityProperty(dbValue int64) (goValue time.Time, err error) {
	err = goValue.UnmarshalText([]byte(dbValue))
	if err != nil {
		err = fmt.Errorf("error unmarshalling time %v: %v", dbValue, err)
	}
	return goValue, err
}
​
// converts time.Time to Unix timestamp in milliseconds 
// i. e. internal format expected by ObjectBox on a date field
func timeInt64ToDatabaseValue(goValue time.Time) (int64, error) {
	var ms = int64(goValue.Nanosecond()) / 1000000
	return goValue.Unix()*1000 + ms, nil
}

Queries on custom types

When you use a converter, the actual value stored in the database is the result of the ...ToDatabaseValue() call, e.g. int64 in the previous example. Therefore, when you want to compare the stored data in a query condition, make sure you use the converted value as well:

// Create
id, _ := box.Put(&model.Task{
	Text: "Buy milk",
	DateCreated: time.Now().UTC()
})
​
// Query
minTime, _ := time.Parse(time.RFC3339, "2018-11-28T12:16:42.145+07:00")
minTimeInt64, _ := objectbox.TimeInt64ConvertToDatabaseValue(minTime)
tasks, _ := box.Query(
	model.Task_.DateCreated.GreaterThan(minTimeInt64)
).Find()

Things to look out for

You must not interact with the database (such as using Box or ObjectBox) inside the converter. The converter methods are called within a transaction, so for example getting or putting entities to a box will fail.

Your converter implementation must be thread safe as it can be called from multiple go routines in parallel. Try to avoid using global variables.

Query is unaware of custom types. You have to use the primitive DB type for queries.