How to save nested List<String> in RoomDB on Android

问题

Hey google have an example on using @Relation

@Entity
 public class Pet {
     int userId;
     String name;
     // other fields
 }
 public class UserNameAndAllPets {
   public int id;
   public String name;
   @Relation(parentColumn = "id", entityColumn = "userId")
   public List<Pet> pets;
 }

Is it possible to save list of String without creating extra class for it. I want to avoid incosistence between my JsonProperty and a room Entity

W would like to have soemthing like that

 public class UserNameAndAllPets {
   @JsonProperty("id")
   public int id;
   @JsonProperty("name")
   public String name;
   @Relation(parentColumn = "id")
   @JsonProperty("pets")
   public List<String> pets;
 }

Because I receive following Json:

{ 
 "id" : "1",
"name" : "someName",
"pets": ["cat", "dog", "camel"]
}

Any one know the solution?

EDIT:

now my example code looks like But I've got Error: Error:Type of the parameter must be a class annotated with @Entity or a collection/array of it.

@JsonIgnoreProperties(ignoreUnknown=true)
@Entity(tableName = TABLE_NAME)
public class Item {
    @Ignore public static final String TABLE_NAME = "itemTable";

    @PrimaryKey(autoGenerate = true)
    Long id;
    @JsonProperty("supplierName")
    String supplierName;
    @JsonProperty("eventDescription")
    String eventDescription;
    @JsonProperty("eventDate")
    @TypeConverters(StringListToGsonConverter.class)
    Date date;
    @JsonProperty("carServiceType")
    @TypeConverters(StringListToGsonConverter.class)
    List<String> types;

    public ServiceHistoryItem(Long id, String supplierName, String eventDescription, Date date, List<String> types) {
        this.id = id;
        this.supplierName = supplierName;
        this.eventDescription = eventDescription;
        this.date = date;
        this.types = types;
    }

    public static class StringListToGsonConverter{
        @TypeConverter
        public static List<String> restoreList(String listOfString){
            return new Gson().fromJson(listOfString, new TypeToken<List<String>>() {}.getType());
        }

        @TypeConverter
        public static String saveListOfString(List<String> listOfString){
            return new Gson().toJson(listOfString);
        }

        @TypeConverter
        public static Date fromTimestamp(Long value) {
            return value == null ? null : new Date(value);
        }

        @TypeConverter
        public static Long dateToTimestamp(Date date) {
            return date == null ? null : date.getTime();
        }
    }
}

EDIT2

New Issue arise when saving items Dao cannot insert List of My Entities, no reason Why... though Error:Type of the parameter must be a class annotated with @Entity or a collection/array of it.

@Dao
interface ItemDao {
    @Query("SELECT * FROM " + Item.TABLE_NAME)
    fun getAll(): LiveData<List<Item>>

    @Query("DELETE FROM " + Item.TABLE_NAME)
    fun deleteAllServiceHistory()

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertNewItem(item: Item)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertNewItems(itemList: List<Item>) // <--- Error

}

SOLUTION for Dao

if you are using Kotlin you should use ArrayList

@Insert(onConflict = OnConflictStrategy.REPLACE)
        fun insertNewItems(itemList: ArrayList<Item>)

回答1:

I had the same issue, which I solved with @TypedConverter. I am saving the list as JSONArray.toString in the db.

@TypeConverter
public static List<String> restoreList(String listOfString) {
    return new Gson().fromJson(listOfString, new TypeToken<List<String>>() {}.getType());
}

@TypeConverter
public static String saveList(List<String> listOfString) {
    return new Gson().toJson(listOfString);
}

This way every List<String> will be serialized as a JSONArray in your db.

To your db class, the one that extends RoomDatabase, you will have to declare which class to use for this conversion with @TypeConverters(Converters.class). E.g.

@Database(version = 1, entities = {Entity.class})
@TypeConverters(Converters.class)
public abstract class MoviesDatabase extends RoomDatabase {

来源：https://stackoverflow.com/questions/45477889/how-to-save-nested-liststring-in-roomdb-on-android