Android会议室数据库：如何处理实体中的Arraylist？ [英] Android Room Database: How to handle Arraylist in an Entity?

问题描述

我刚刚实施了Room以进行离线数据保存。但是在Entity类中，我收到以下错误：

I just implemented Room for offline data saving. But in an Entity class, I am getting the following error:

Error:(27, 30) error: Cannot figure out how to save this field into database. You can consider adding a type converter for it.

该课程如下：

@Entity(tableName = "firstPageData")
public class MainActivityData {
@PrimaryKey
private String userId;

@ColumnInfo(name = "item1_id")
private String itemOneId;

@ColumnInfo(name = "item2_id")
private String itemTwoId;

   // THIS IS CAUSING THE ERROR... BASICALLY IT ISN'T READING ARRAYS
   @ColumnInfo(name = "mylist_array")
    private ArrayList<MyListItems> myListItems;

public String getUserId() {
    return userId;
}

public void setUserId(String userId) {
    this.userId = userId;
}


public ArrayList<MyListItems> getMyListItems() {
    return myListItems;
}

public void setCheckListItems(ArrayList<MyListItems> myListItems) {
    this.myListItems = myListItems;
}
}

所以基本上我想将ArrayList保存在数据库中但我无法找到与之相关的任何内容。你可以指导我如何使用Room保存数组吗？

So basically I want to save the ArrayList in the database but I was not able to find anything relevant to it. Can you guide me regarding how to save an Array using Room?

注意：MyListItems Pojo类包含2个字符串（截至目前）

NOTE: MyListItems Pojo class contains 2 Strings (as of now)

提前致谢。

推荐答案

选项＃1： MyListItems 是 @Entity ，因为 MainActivityData 是。 MyListItems 会将 @ForeignKey 设置回 MainActivityData 。但是，在这种情况下， MainActivityData 不能有 private ArrayList< MyListItems> myListItems ，如在Room中，实体不引用其他实体。但是，视图模型或类似的POJO构造可以具有 MainActivityData 及其关联的 ArrayList< MyListItems> 。

Option #1: Have MyListItems be an @Entity, as MainActivityData is. MyListItems would set up a @ForeignKey back to MainActivityData. In this case, though, MainActivityData cannot have private ArrayList<MyListItems> myListItems, as in Room, entities do not refer to other entities. A view model or similar POJO construct could have a MainActivityData and its associated ArrayList<MyListItems>, though.

选项＃2：设置一对 @TypeConverter 方法来转换 ArrayList< MyListItems> 来往于某些基本类型（例如， String ，例如使用JSON作为存储格式）。现在， MainActivityData 可以直接使用 ArrayList< MyListItems> 。但是， MyListItems 将没有单独的表，因此您无法在 MyListItems 上查询。

Option #2: Set up a pair of @TypeConverter methods to convert ArrayList<MyListItems> to and from some basic type (e.g., a String, such as by using JSON as a storage format). Now, MainActivityData can have its ArrayList<MyListItems> directly. However, there will be no separate table for MyListItems, and so you cannot query on MyListItems very well.

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