Android Room Persistence Library: Upsert

问题:

Android's Room persistence library graciously includes the @Insert and @Update annotations that work for objects or collections. I however have a use case (push notifications containing a model) that would require an UPSERT as the data may or may not exist in the database.

Sqlite doesn't have upsert natively, and workarounds are described in this SO question. Given the solutions there, how would one apply them to Room?

To be more specific, how can I implement an insert or update in Room that would not break any foreign key constraints? Using insert with onConflict=REPLACE will cause the onDelete for any foreign key to that row to be called. In my case onDelete causes a cascade, and reinserting a row will cause rows in other tables with the foreign key to be deleted. This is NOT the intended behavior.

回答1:

For more elegant way to do that I would suggest two options:

Checking for return value from insert operation with IGNORE as a OnConflictStrategy (if it equals to -1 then it means row wasn't inserted):

@Insert(onConflict = OnConflictStrategy.IGNORE)
long insert(Entity entity);

@Update(onConflict = OnConflictStrategy.IGNORE)
void update(Entity entity);

public void upsert(Entity entity) {
    long id = insert(entity);
    if (id == -1) {
        update(entity);   
    }
}

Handling exception from insert operation with FAIL as a OnConflictStrategy:

@Insert(onConflict = OnConflictStrategy.FAIL)
void insert(Entity entity);

@Update(onConflict = OnConflictStrategy.FAIL)
void update(Entity entity);

public void upsert(Entity entity) {
    try {
        insert(entity);
    } catch (SQLiteConstraintException exception) {
        update(entity);
    }
}

回答2:

I could not find a SQLite query that would insert or update without causing unwanted changes to my foreign key, so instead I opted to insert first, ignoring conflicts if they occurred, and updating immediately afterwards, again ignoring conflicts.

The insert and update methods are protected so external classes see and use the upsert method only. Keep in mind that this isn't a true upsert as if any of the MyEntity POJOS have null fields, they will overwrite what may currently be in the database. This is not a caveat for me, but it may be for your application.

@Insert(onConflict = OnConflictStrategy.IGNORE)
protected abstract void insert(List<MyEntity> entities);

@Update(onConflict = OnConflictStrategy.IGNORE)
protected abstract void update(List<MyEntity> entities);

public void upsert(List<MyEntity> entities) {
    insert(models);
    update(models);
}

回答3:

Perhaps you can make your BaseDao like this.

secure the upsert operation with @Transaction, and try to update only if insertion is failed.

@Dao
public abstract class BaseDao<T> {
    /**
    * Insert an object in the database.
    *
     * @param obj the object to be inserted.
     * @return The SQLite row id
     */
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    public abstract long insert(T obj);

    /**
     * Insert an array of objects in the database.
     *
     * @param obj the objects to be inserted.
     * @return The SQLite row ids   
     */
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    public abstract List<Long> insert(List<T> obj);

    /**
     * Update an object from the database.
     *
     * @param obj the object to be updated
     */
    @Update
    public abstract void update(T obj);

    /**
     * Update an array of objects from the database.
     *
     * @param obj the object to be updated
     */
    @Update
    public abstract void update(List<T> obj);

    /**
     * Delete an object from the database
     *
     * @param obj the object to be deleted
     */
    @Delete
    public abstract void delete(T obj);

    @Transaction
    public void upsert(T obj) {
        long id = insert(obj);
        if (id == -1) {
            update(obj);
        }
    }

    @Transaction
    public void upsert(List<T> objList) {
        List<Long> insertResult = insert(objList);
        List<T> updateList = new ArrayList<>();

        for (int i = 0; i < insertResult.size(); i++) {
            if (insertResult.get(i) == -1) {
                updateList.add(objList.get(i));
            }
        }

        if (!updateList.isEmpty()) {
            update(updateList);
        }
    }
}

回答4:

If the table has more than one column, you can use

@Insert(onConflict = OnConflictStrategy.REPLACE)

to replace a row.

Reference - Go to tips Android Room Codelab

回答5:

Just an update for how to do this with Kotlin retaining data of the model (Maybe to use it in a counter as in example):

//Your Dao must be an abstract class instead of an interface (optional database constructor variable)
@Dao
abstract class ModelDao(val database: AppDatabase) {

@Insert(onConflict = OnConflictStrategy.FAIL)
abstract fun insertModel(model: Model)

//Do a custom update retaining previous data of the model 
//(I use constants for tables and column names)
 @Query("UPDATE $MODEL_TABLE SET $COUNT=$COUNT+1 WHERE $ID = :modelId")
 abstract fun updateModel(modelId: Long)

//Declare your upsert function open
open fun upsert(model: Model) {
    try {
       insertModel(model)
    }catch (exception: SQLiteConstraintException) {
        updateModel(model.id)
    }
}
}

You can also use @Transaction and database constructor variable for more complex transactions using database.openHelper.writableDatabase.execSQL("SQL STATEMENT")

回答6:

Another approach I can think of is to get the entity via DAO by query, and then perform any desired updates. This may be less efficient compared to the other solutions in this thread in terms of runtime because of having to retrieve the full entity, but allows much more flexibility in terms of operations allowed such as on what fields/variable to update.

For example :

private void upsert(EntityA entityA) {
   EntityA existingEntityA = getEntityA("query1","query2");
   if (existingEntityA == null) {
      insert(entityA);
   } else {
      entityA.setParam(existingEntityA.getParam());
      update(entityA);
   }
}

回答7:

Should be possible with this sort of statement:

INSERT INTO table_name (a, b) VALUES (1, 2) ON CONFLICT UPDATE SET a = 1, b = 2