When working with Room and Kotlin, you may encounter the error “Cannot figure out how to save this field into database”. This issue typically arises when you are trying to store a complex data type, such as a List or an Enum, directly into the database. In this blog post, we’ll explore how to resolve this error using TypeConverters in Room Kotlin. Additionally, we will discuss how to create a custom converter for a List to JSON string conversion.
Understanding the Error
Room does not support storing Lists or Enums directly, and hence the need for TypeConverters. TypeConverters are used to convert complex data types into simple ones that Room can store, and vice versa. In this post, we will focus on converting a List of custom objects into a JSON string and back.
Creating the Entity and Converter
First, let’s take a look at our CountryModel entity:
@Entity(tableName = TABLE_NAME)
public class CountryModel {
public static final String TABLE_NAME = "Countries";
@PrimaryKey
private int idCountry;
/* I WANT TO CONVERT THIS LIST TO A JSON STRING */
private List countryLang = null;
// Getters and setters
}
Now, let’s create a Converters class to handle the conversion of the List to a JSON string:
public class Converters {
@TypeConverter
public static String countryLangToJson(List list) {
if(list == null)
return null;
CountryLang lang = list.get(0);
return list.isEmpty() ? null : new Gson().toJson(lang);
}
}
Registering the TypeConverter
To register our TypeConverter, we’ll add the @TypeConverters annotation to our RoomDatabase class:
@Database(entities = {CountryModel.class}, version = 1 ,exportSchema = false)
@TypeConverters({Converters.class})
public abstract class MYDatabase extends RoomDatabase {
public abstract CountriesDao countriesDao();
}
Error Resolution and Additional Considerations
- Ensure that you have a converter for both directions (List to JSON string and JSON string to List).
- Make sure the @TypeConverters annotation is added to the RoomDatabase class.
- For Enums, you can create a similar TypeConverter that converts the Enum to a String (or Integer) and back.
- Don’t forget to update your RoomDatabase version after making changes.
Conclusion
By using TypeConverters in Room Kotlin, you can easily store complex data types, such as Lists or Enums, into your database. This approach not only resolves the error “Cannot figure out how to save this field into database” but also makes your code more robust and maintainable. Keep exploring and happy coding!
