Android机房持久性库-TypeConverter错误错误:无法弄清楚如何将字段保存到数据库 [英] Android room persistent library - TypeConverter error of error: Cannot figure out how to save field to database"

问题描述

由于错误，我无法在房间中创建typeConverter.我似乎正在按照文档进行所有操作.我想将列表转换为json字符串.让我们来看看我的实体:

Im not able to create a typeConverter in room due to an error. I seem to be following everything per the docs. I would like to convert a list to a json string. lets take a look at my 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> 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;
    }

    public List<CountryLang> getCountryLang() {
        return countryLang;
    }

    public void setCountryLang(List<CountryLang> countryLang) {
        this.countryLang = countryLang;
    }

}

country_lang 是我想要转换为字符串json的内容.所以我创建了以下转换器: Converters.java:

The country_lang is what i would like to convert to a string json. So i created the following converter: Converters.java:

public class Converters {

@TypeConverter
public static String countryLangToJson(List<CountryLang> list) {

    if(list == null)
        return null;

        CountryLang lang = list.get(0);

    return list.isEmpty() ? null : new Gson().toJson(lang);
}}

然后问题出在我放置@TypeConverters({Converters.class})的任何地方，我一直收到错误消息.但是正式地，这是我放置注释以注册typeConverter的地方:

then the problem is anywhere i put the @TypeConverters({Converters.class}) i keep getting an error. But officially this is where i have placed the annotation to get the typeConverter registered:

@Database(entities = {CountryModel.class}, version = 1 ,exportSchema = false)
@TypeConverters({Converters.class})
public abstract class MYDatabase extends RoomDatabase {
    public abstract CountriesDao countriesDao();
}

我得到的错误是:

Error:(58, 31) error: Cannot figure out how to save this field into database. You can consider adding a type converter for it.

推荐答案

自Room宣布以来，这是我常见的问题. Room不支持直接存储列表的功能，也不支持转换到列表或从列表转换的功能.它支持转换和存储POJO.

This is a common problem I've seen since Room was announced. Room does not support the ability to store Lists directly, nor the ability to convert to/from Lists. It supports converting and storing POJO's.

在这种情况下，解决方案很简单.而不是存储List<CountryLang>，而是要存储CountryLangs(请注意's')

In this case the solution is simple. Instead of storing a List<CountryLang> you want to store CountryLangs (note the 's')

我在这里做了一个简单的解决方案示例:

I've done a quick example of a solution here :

public class CountryLangs {
    private List<String> countryLangs;

    public CountryLangs(List<String> countryLangs) {
        this.countryLangs = countryLangs;
    }

    public List<String> getCountryLangs() {
        return countryLangs;
    }

    public void setCountryLangs(List<String> countryLangs) {
        this.countryLangs = countryLangs;
    }
}

此POJO是您先前对象的反转.它是存储语言列表的对象.而不是存储您的语言的对象列表.

This POJO is an inversion of your previous object. It is an object that stores a list of languages. Instead of a list of objects that store your language.

public class LanguageConverter {
    @TypeConverter
    public CountryLangs storedStringToLanguages(String value) {
        List<String> langs = Arrays.asList(value.split("\\s*,\\s*"));
        return new CountryLangs(langs);
    }

    @TypeConverter
    public String languagesToStoredString(CountryLangs cl) {
        String value = "";

        for (String lang :cl.getCountryLangs())
            value += lang + ",";

        return value;
    }
}

此转换器获取字符串列表，并将其转换为逗号分隔的字符串，以存储在单列中.当它从SQLite数据库中获取字符串以进行转换时，它会以逗号分隔列表，并填充CountryLangs.

This converter takes a list of strings and converts them into a comma seperated string to be stored in a single column. When it fetches the string from the SQLite db to convert back, it splits the list on commas, and populates the CountryLangs.

确保在进行这些更改后更新RoomDatabase版本.其余配置正确.余下的房间持久性工作将使您快乐地狩猎.

Insure to update your RoomDatabase version after making these changes.You have the rest of the configuration correct. Happy hunting with the rest of your Room persistence work.

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