房间数据库迁移：java.lang.IllegalStateException：迁移未正确处理 <table_name>

Question

我正在尝试使用房间数据库创建一个新表。我遵循了与程序中已有的第一个表相同的原则，但是当运行应用程序时，我收到此错误：

Caused by: java.lang.IllegalStateException: Migration didn't properly handle: WaterFountainEntry(com.mpms.relatorioacessibilidadecortec.entities.WaterFountainEntry).
     Expected:
    TableInfo{name='WaterFountainEntry', columns={waterFountainHeight=Column{name='waterFountainHeight', type='REAL', affinity='4', notNull=false, primaryKeyPosition=0, defaultValue='null'}, waterFountainID=Column{name='waterFountainID', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=1, defaultValue='null'}, cupHolderHeight=Column{name='cupHolderHeight', type='REAL', affinity='4', notNull=false, primaryKeyPosition=0, defaultValue='null'}, schoolEntryID=Column{name='schoolEntryID', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=0, defaultValue='null'}, waterFountApproximation=Column{name='waterFountApproximation', type='REAL', affinity='4', notNull=false, primaryKeyPosition=0, defaultValue='null'}}, foreignKeys=[ForeignKey{referenceTable='SchoolEntry', onDelete='CASCADE', onUpdate='CASCADE', columnNames=[schoolEntryID], referenceColumnNames=[cadID]}], indices=[]}
     Found:
    TableInfo{name='WaterFountainEntry', columns={waterFountainID=Column{name='waterFountainID', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=1, defaultValue='null'}, waterFountainHeight=Column{name='waterFountainHeight', type='DOUBLE', affinity='4', notNull=true, primaryKeyPosition=0, defaultValue='null'}, cupHolderHeight=Column{name='cupHolderHeight', type='DOUBLE', affinity='4', notNull=true, primaryKeyPosition=0, defaultValue='null'}, schoolEntryID=Column{name='schoolEntryID', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=0, defaultValue='null'}, waterFountApproximation=Column{name='waterFountApproximation', type='DOUBLE', affinity='4', notNull=true, primaryKeyPosition=0, defaultValue='null'}}, foreignKeys=[ForeignKey{referenceTable='SchoolEntry', onDelete='NO ACTION', onUpdate='NO ACTION', columnNames=[schoolEntryID], referenceColumnNames=[cadID]}], indices=[]}

通过阅读此错误消息，我确实收集到该程序期望某些字段具有不同的类型（也可能与它们的顺序不同），但问题是它不应该是这样的。我确保完全按照我在实体中放置字段的方式编写迁移类，如下所示：

<Migration Class and Database Creation>

 static final Migration MIGRATION_2_3 = new Migration(2,3) {
        @Override
        public void migrate(@NonNull SupportSQLiteDatabase database) {
            database.execSQL("CREATE TABLE WaterFountainEntry (waterFountainID INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL," +
                    "schoolEntryID INTEGER NOT NULL, waterFountainHeight DOUBLE NOT NULL, cupHolderHeight DOUBLE NOT NULL," +
                    "waterFountApproximation DOUBLE NOT NULL, FOREIGN KEY (schoolEntryID) REFERENCES SchoolEntry (cadID))");
        }
    };

    public static ReportDatabase getDatabase(final Context context) {
        if (INSTANCE == null){
            synchronized (ReportDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(), ReportDatabase.class, "ReportDatabase")
                            .addCallback(roomCallback).addMigrations(MIGRATION_2_3).build();
                }
            }
        }
        return INSTANCE;
    }

<WaterFountainEntry entity>

@Entity(foreignKeys = @ForeignKey(entity = SchoolEntry.class, parentColumns = "cadID", childColumns = "schoolEntryID",
        onDelete = CASCADE, onUpdate = CASCADE))
public class WaterFountainEntry {

    @PrimaryKey(autoGenerate = true)
    @NonNull
    private Integer waterFountainID;
    @NonNull
    private Integer schoolEntryID;
    @NonNull
    private Double waterFountainHeight;
    @NonNull
    private Double cupHolderHeight;
    @NonNull
    private Double waterFountApproximation;

/** getters & setters **/

    public WaterFountainEntry(@NonNull Integer schoolEntryID, @NonNull Double waterFountainHeight,
                              @NonNull Double cupHolderHeight, @NonNull Double waterFountApproximation) {
        this.schoolEntryID = schoolEntryID;
        this.waterFountainHeight = waterFountainHeight;
        this.cupHolderHeight = cupHolderHeight;
        this.waterFountApproximation = waterFountApproximation;
    }

那么，可能是什么错误呢？我真的看了很多文档也找不到这个问题的原因。

Answer 1

获取正确 SQL 的简单技巧是让 Room 代表您生成 SQL。

创建实体，然后编译（Cntrl+F9），然后查看生成的java（从Android视图），然后查找与@Database类相同且后缀为_Impl的文件，然后找到createAllTables方法并SQL就在那里。这正是 Room 所期望的。

例如：-