Android room persistent library - how to insert class that has a List object field

Question

In Android room persistent library how to insert entire Model object into table which has in itself another list.

Let me show you what i mean :

@Entity(tableName = TABLE_NAME) public class CountryModel {      public static final String TABLE_NAME = "Countries";      @PrimaryKey     private int idCountry;      private List<CountryLang> countryLang = null;      public int getIdCountry() {         return idCountry;     }      public void setIdCountry(int idCountry) {         this.idCountry = idCountry;     }      public String getIsoCode() {         return isoCode;     }      public void setIsoCode(String isoCode) {         this.isoCode = isoCode;     }      /**          here i am providing a list of coutry information how to insert          this into db along with CountryModel at same time      **/     public List<CountryLang> getCountryLang() {         return countryLang;     }      public void setCountryLang(List<CountryLang> countryLang) {         this.countryLang = countryLang;     } } 

my DAO looks like this:

@Dao public interface CountriesDao{      @Query("SELECT * FROM " + CountryModel.TABLE_NAME +" WHERE isoCode =:iso_code LIMIT 1")     LiveData<List<CountryModel>> getCountry(String iso_code);      @Query("SELECT * FROM " + CountryModel.TABLE_NAME )     LiveData<List<CountryModel>> getAllCountriesInfo();      @Insert(onConflict = REPLACE)     Long[] addCountries(List<CountryModel> countryModel);      @Delete     void deleteCountry(CountryModel... countryModel);      @Update(onConflict = REPLACE)     void updateEvent(CountryModel... countryModel); } 

When i call database.CountriesDao().addCountries(countryModel); i get the following room db compile error: Error:(58, 31) error: Cannot figure out how to save this field into database. You can consider adding a type converter for it.

should there be another table called CountryLang ? and if so how to tell room to connect them on insert statement ?

The CountryLang object itself looks like this:

public class CountryLang {       private int idCountry;      private int idLang;      private String name;      public int getIdCountry() {         return idCountry;     }      public void setIdCountry(int idCountry) {         this.idCountry = idCountry;     }      public int getIdLang() {         return idLang;     }      public void setIdLang(int idLang) {         this.idLang = idLang;     }      public String getName() {         return name;     }      public void setName(String name) {         this.name = name;     }  } 

the response looks like this:

"country_lang": [       {         "id_country": 2,         "id_lang": 1,         "name": "Austria"       }     ] 

For every country so its not going to be more then one item here. Im comfortable desgning it for just one item in the country_lang list. So i can just make a table for country_lang and then some how link it to CountryModel. but how ? can i use foreign key ? i was hoping i did not have to use a flat file. so your saying i have to store it as json ? Is it recommended not to use room for temporary ? what to use instead ?

Answer 1

You can easly insert the class with list object field using TypeConverter and GSON,

public class DataConverter {      @TypeConverter     public String fromCountryLangList(List<CountryLang> countryLang) {         if (countryLang == null) {             return (null);         }         Gson gson = new Gson();         Type type = new TypeToken<List<CountryLang>>() {}.getType();         String json = gson.toJson(countryLang, type);         return json;     }      @TypeConverter     public List<CountryLang> toCountryLangList(String countryLangString) {         if (countryLangString == null) {             return (null);         }         Gson gson = new Gson();         Type type = new TypeToken<List<CountryLang>>() {}.getType();         List<CountryLang> countryLangList = gson.fromJson(countryLangString, type);         return countryLangList;     }  } 

Next, Add the @TypeConverters annotation to the AppDatabase class

    @Database(entities = {CountryModel.class}, version = 1)     @TypeConverters({DataConverter.class})     public abstract class AppDatabase extends RoomDatabase {       public abstract CountriesDao countriesDao();     } 

For more information about TypeConverters in Room check our blog here and the official docs.