Android房间外键约束失败787。文件夹系统

Question

How can I realize folder system for db in Room? I want create folders and store in this some data. Nested folders is not necessary.

Before I use table with data (webSites) and folders table, then I create 3rd table WebSitesWithFolders, where I storing webSites_id and folders_id. But when I update or delete field in users table all working good, but if I delete field from Folder, then I getting error foreign KEY constraint failed (code 787).

WebSites Entity

@Entity
public class WebSites {

@PrimaryKey(autoGenerate = true)
private int id;

@ColumnInfo()
private String website_head;

@ColumnInfo()
private String website_url;

@ColumnInfo()
private String img_src;


public WebSites(@NonNull String website_head, String website_url, String img_src) {
    this.website_head = website_head;
    this.website_url = website_url;
    this.img_src = img_src;
}


public int getId() {
    return id;
}

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


public String getWebsite_head() {
    return website_head;
}

public void setWebsite_head(String website_head) {
    this.website_head = website_head;
}

public String getWebsite_url() {
    return website_url;
}

public void setWebsite_url(String website_url) {
    this.website_url= website_url;
}

public String getImg_src() {
    return img_src;
}

public void setImg_src(String img_src) {
    this.img_src = img_src;
}
}

Folders Entity

@Entity
public class Folders {


@PrimaryKey(autoGenerate = true)
private int id;

@ColumnInfo()
private String name;

public Folders(String name) {
    this.name = name;
}

public int getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

}

WebSitesWithFolders

@Entity(tableName = "websites_with_folders",
    foreignKeys = {
            @ForeignKey(entity = WebSites.class,
                        parentColumns = "id",
                        childColumns = "webSitesId",
                        onDelete = CASCADE,
                        onUpdate = CASCADE),
            @ForeignKey(entity = Folders.class,
                        parentColumns = "id",
                        childColumns = "foldersId",
                        onDelete = CASCADE,
                        onUpdate = CASCADE)
            })

public class WebSitesWithFolders {

@PrimaryKey(autoGenerate = true)
private int idd;

@ColumnInfo()
private int webSitesId;

@ColumnInfo()
private int foldersId;

public WebSitesWithFolders(int webSitesId, int foldersId) {
    this.webSitesId = webSitesId;
    this.foldersId = foldersId;
}

public int getWebSitesId() {
    return webSitesId;
}

public void setWebSitesId(int webSitesId) {
    this.webSitesId = webSitesId;
}

public int getFoldersId() {
    return foldersId;
}

public void setFoldersId(int foldersId) {
    this.foldersId = foldersId;
}

public int getIdd() {
    return idd;
}

public void setIdd(int idd) {
    this.idd = idd;
}
}

Answer 1

In my experience you get the 787 foreign key constraint failed in Room when some ids that you declared in the relationship are missing. My mistake was that I was trying to add an object in a third table (for a many to many relationship) before adding the object in one of the parent tables.

To wrap up, make sure that on delete you're not remaining in an inconsistent state (websites_with_folders table has ids that are no longer present in the website or folder tables). Having cascade property on delete should help you with that, but I would double check what's happening there with a test. Also, I suggest that instead of using an "id" primary key for the websites_with_folders you should use multiple primary keys consisting of webSitesId and foldersId, like this:

@Entity(tableName = "websites_with_folders", 
        primaryKeys ={webSitesId ,foldersId}

Hope this solves your problem!