使用Android Room从Junction Table获取数据 [英] Get data from Junction Table with Android Room

问题描述

我目前正在构建一个Android应用，该应用显示由多个航路点构成的Route.我已经计划了数据库模式(chen表示法[可能无效的语法"]):

I am currently building an android app, which displays a Route, which is constructed out of multiple waypoints. I already planned the database schema (chen-notation [possibly invalid "syntax"]):

我试图重新创建与Android房间的n-m关系，但是我不知道如何检索联结表(route_waypoint)的 index_of_route 属性.

I tried to recreate the n-m relation with android room, but I can't figure out how I can retrieve the index_of_route attribute of the junction table (route_waypoint).

当我像这样获取数据时，我想要联结表属性 index_of_route :

I want the junction table attribute index_of_route, when I get the Data like so:

    @Transaction
    @Query("SELECT * FROM POIRoute")
    List<RouteWithWaypoints> getRoutes();

在POIWaypoint类内部(可能是额外的属性)，或者至少可以从另一个类进行访问，该类可以这样实现:

inside the POIWaypoint class (maybe as extra attribute), or at least accessible from another class which maybe is implemented like so:

    @Embedded
    POIWaypoint waypoint;
    int indexOfRoute;

目前，我没有从联结表中获得 indexOfRoute 属性.

Currently I don't get the indexOfRoute attribute from the junction table.

我已经创建的类:

RouteWithWaypoints:

RouteWithWaypoints:

public class RouteWithWaypoints {

    @Embedded
    private POIRoute poiRoute;

    @Relation(parentColumn = "id",entityColumn = "id",associateBy = @Junction(value = RouteWaypoint.class, parentColumn = "routeId", entityColumn = "waypointId"))
    private List<POIWaypoint> waypoints;

    public POIRoute getPoiRoute() {
        return poiRoute;
    }

    public void setPoiRoute(POIRoute poiRoute) {
        this.poiRoute = poiRoute;
    }

    public List<POIWaypoint> getWaypoints() {
        return waypoints;
    }

    public void setWaypoints(List<POIWaypoint> waypoints) {
        this.waypoints = waypoints;
    }

RouteWaypoint:

RouteWaypoint:

@Entity(primaryKeys = {"waypointId", "routeId"}, foreignKeys = {
        @ForeignKey(entity = POIWaypoint.class, parentColumns = {"id"}, childColumns = {"waypointId"}),
        @ForeignKey(entity = POIRoute.class, parentColumns = {"id"}, childColumns = {"routeId"})
})
public class RouteWaypoint {
    private int waypointId;
    private int routeId;
    
    // I want this attribute inside the POIWaypoint class
    @ColumnInfo(name = "index_of_route")
    private int indexOfRoute;

    public int getWaypointId() {
        return waypointId;
    }

    public void setWaypointId(int waypointId) {
        this.waypointId = waypointId;
    }

    public int getRouteId() {
        return routeId;
    }

    public void setRouteId(int routeId) {
        this.routeId = routeId;
    }
}

POIRoute:

@Entity
public class POIRoute{

    private String name;
    private String description;
    @PrimaryKey(autoGenerate = true)
    private int id;
    private boolean user_generated;
    private int parentId;


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public boolean isUser_generated() {
        return user_generated;
    }

    public void setUser_generated(boolean user_generated) {
        this.user_generated = user_generated;
    }

    public int getParentId() {
        return parentId;
    }

    public void setParentId(int parentId) {
        this.parentId = parentId;
    }


}

POIWaypoint(请忽略尚未完成的position属性):

POIWaypoint (please ignore the position attribute it isn't finished):

@Entity
public class POIWaypoint {
    @PrimaryKey(autoGenerate = true)
    private long id;
    @ColumnInfo(name = "long_description")
    private String longDescription;
    private String title;
    @ColumnInfo(name = "short_description")
    private String shortDescription;

    // use converter: https://developer.android.com/training/data-storage/room/referencing-data
    @Ignore
    private GeoPoint position;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public GeoPoint getPosition() {
        return position;
    }

    public void setPosition(GeoPoint position) {
        this.position = position;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getShortDescription() {
        return shortDescription;
    }

    public void setShortDescription(String shortDescription) {
        this.shortDescription = shortDescription;
    }

    public String getLongDescription() {
        return longDescription;
    }

    public void setLongDescription(String longDescription) {
        this.longDescription = longDescription;
    }

推荐答案

我通过自己管理关系解决了我的问题.我将RouteDao更改为一个抽象类，以插入我自己的方法，该方法自己管理联结表的一部分:

I solved my problem by manage the relation by myself. I changed my RouteDao to an abstract class to insert my own method, which manages part of the junction table by itself:

RouteDao:

 private RouteDatabase database;

    public RouteDao(RouteDatabase database) {
        this.database = database;
    }

    @Query("Select * from POIRoute")
    public abstract List<POIRoute> getRoutes();

    @Query("SELECT * FROM POIRoute WHERE id = :id")
    public abstract POIRoute getRoute(int id);

    @Insert
    abstract void insertRouteWithWaypoints(RouteWithWaypoints routeWithWaypoints);

    public List<RouteWithWaypoints> getRoutesWithWaypoints() {
        List<POIRoute> routes = this.getRoutes();
        List<RouteWithWaypoints> routesWithWaypoints = new LinkedList<>();
        for (POIRoute r : routes) {
            routesWithWaypoints.add(new RouteWithWaypoints(r, database.wayPointDao().getWaypointsFromRoute(r.getId())));
        }

        return routesWithWaypoints;
    }

    public RouteWithWaypoints getRouteWithWaypoints(int id) {
        POIRoute route = this.getRoute(id);
        RouteWithWaypoints routeWithWaypoints = null;
        if (route != null) {
            routeWithWaypoints = new RouteWithWaypoints(route, database.wayPointDao().getWaypointsFromRoute(route.getId()));
        }


        return routeWithWaypoints;
    }

WayPointDao:

WayPointDao:

    @Query("SELECT * FROM POIWaypoint")
    POIWaypoint getWaypoints();

    @Query("SELECT * FROM POIWaypoint WHERE id = :id")
    POIWaypoint getWaypoint(long id);

    @Query("SELECT pw.*, rw.index_of_route as 'index' FROM POIWaypoint as pw Join RouteWaypoint as rw on (rw.waypointId = pw.id) where rw.routeId = :id order by 'index' ASC")
    List<POIRouteStep> getWaypointsFromRoute(int id);

这篇关于使用Android Room从Junction Table获取数据的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！