GreenDAO on Android: Byte Array as Primary Key / Building Query Containing Where Clause on Byte Array Property

Question

I am currently trying to select entries using a query containing a where condition on a (byte array) property. This property/column contains a serialized UUID. Unfortunately I currently cannot change the data type of this column, as the database is created and synced by a separate module which only works properly with the current implementation. As greenDao cannot handle byte arrays as primary keys properly I am trying to work around this issue somehow. Creating my own selection queries etc. would be a solution The property is defined during greenDAO generation as:

Entity randomEntity = schema.addEntity("RandomEntity");
...
randomEntity.addByteArrayProperty("RandomProperty"); 

The query is built using following line:

Query query = this.mRandomEntityDao.queryBuilder().where(RandomEntityDao.Properties.RandomProperty.eq(randomByteArray)).build();

Unfortunately I get following error with this operation:

de.greenrobot.dao.DaoException: Illegal value: found array, but simple object required
at de.greenrobot.dao.query.WhereCondition$PropertyCondition.checkValueForType(WhereCondition.java:75)
...

Is this where condition is simply not supported by greenDAO or am I missing something crucial? Unfortunately I cannot use another datatype for this certain property.

Edit:

My current workaround goes as follows:
As greenDao can handle primary keys which are strings (up to a certain point) and UUIDs can also be represented by this data type I altered the existing tables and added following columns:

db.execSQL("ALTER TABLE 'RANDOMTABLE' ADD COLUMN '_GREENID' TEXT;");

The sync module is ignoring this column so there shouldn’t be any issues with that. Then I created a trigger mapping the serialized UUID in the ID column to the new _GREENID column:

db.execSQL("CREATE TRIGGER randomtableGreen AFTER INSERT ON 'RANDOMTABLE' BEGIN " + 
"UPDATE 'RANDOMTABLE' SET '_GREENID' = HEX(NEW.ID) WHERE 'RANDOMTABLE'.ID = NEW.ID; " +
"END;");

Finally I run an update on the table in case they already contained some entries prior to the creation of the trigger:

db.execSQL("UPDATE 'RANDOMTABLE' SET '_GREENID' = HEX(ID) WHERE '_GREENID' <> '';");

Answer 1

About byte[] aka BLOB in greendao:

Looking at de.greenrobot.dao.query.WhereCondition.PropertyCondition.checkValueForType conditions for byte[] are not supported at the moment, because the following lines will always throw an exception if value is of type byte[].

if (value != null && value.getClass().isArray()) {
    throw new DaoException("Illegal value: found array, but simple object required");
}

Solution 1 - modify and contribute to greendao:

You could modify the uper lines, so that the exception is only thrown if the type of value and the type of the Property don't fit.

if (value != null) {
    if (value.getClass().isArray() && !property.type.isArray()) {
        throw new DaoException("Illegal value: found array, but " +
                "simple object required");
    }
    if (!value.getClass().isArray() && property.type.isArray()) {
        throw new DaoException("Illegal value: found simple object, " +
                "but array required");
    }
}

Maybe this will already solve the problem, but there may be other parts in greendao stopping to work with this edit or that will break the query. For example the binding of parameters to queries may not work with arrays.

Solutinon 2 - Use queryRaw(String where, String... selectionArg)

This is pretty straight forward and shouldn't be a problem with some knowledge about SQLite.

Solution 3 - Using a lookup-table

Assume the original table:

ORIG
-------------------------------
UUID     BLOB
...

You can modify ORIG and add a autoincrement-primarykey:

db.execSQL("ALTER TABLE 'ORIG' " +
        "ADD COLUMN 'REF_ID' INT PRIMARYKEY AUTOINCREMENT;");

The sync service should already take care about the uniqueness of ORIG.UUID and ignore the new ORIG.REF_ID-column. For inserting new UUIDs the sync service will probably use INSERT causing a new autoincremented value in ORIG.REF_ID. For updating an existing UUID the sync service will probably use UPDATE ... WHERE UUID=? and no new ORIG.REF_ID-value will be created, but the old value will be kept.

Summarized the ORIG-table has a new Bijection between column REF_ID and column UUID.

Now you can create another table:

ORIG_IDX
------------------------------
UUID      TEXT PRIMARYKEY
REF_ID    INT  UNIQUE

(If your data is smaller than 8 bytes it will also fit into a INT instead TEXT, but I don't know if there is a built-in cast/coversion from BLOB to INT.)

ORIG.IDX.UUID will be the String-representation of ORIG.UUID. ORIG_IDX.REF_ID is foreign key for ORIG.REF_ID.

ORIG_IDX is filled and updated by triggers:

db.execSQL("CREATE TRIGGER T_ORIG_AI AFTER INSERT ON 'ORIG' BEGIN " + 
        "INSERT 'ORIG_IDX' SET 'REF_ID' = NEW.REF_ID, 'UUID' = NEW.UUID" +
        "END;");

Create corresponding triggers for UPDATE and DELETE.

You can create the tables ORIG and ORIG_IDX using greendao and then query a requested uuid with:

public Orig getOrig(String uuid) {
    OrigIdx origIdx = OrigIdxDao.queryBuilder().where(
            QrigIdxDao.Properties.UUID.eq(uuid)).unique();
    if (origIdx != null) {
        return origIdx.getOrig();
    }
    return null;
}

I think String-primarykey is not supported yet, so dao.load(uuid) won't be available.

CONCERING AN EXTENDED TABLE:

You could use a string primarykey-column and provide conversion-methods in the keep-sections of your entity. You will have to compute the primarykey-column before you do an insert.

If there are other tools inserting data (for example your sync service) you'd have to use a trigger to compute your primary-key before the insert happens. This doesn't seem possible using SQLite. Thus the primarykey-constraint will fail on inserts by the sync service, so this solution will not work with primarykey!