Why is my ForeignCollection always empty?

[Salil Wadnerkar]

Hi,

I have a table readings which has references devices table.

I define the relation in my device class like this:

    @ForeignCollectionField(eager = true, foreignFieldName = "device") # I tried it with default - lazy first, And it did not work.

    ForeignCollection<Reading> readings;

And, in reading class like this:

    @DatabaseField(canBeNull = false, foreign = true, columnName = "device_mac_addr", foreignColumnName = "mac_addr")

    private Device device;

My query code is this:

        List<Device> devices = (List<Device>)deviceDao.queryForEq("mac_addr", deviceMacAddr);

        if (!devices.isEmpty()) {

            ForeignCollection<Reading> readings = devices.get(0).getReadings();

I get the correct device. But, the readings are always empty. Could someone tell me why this is happening?

Thanks

Salil

[Moritz Pfeiffer]

Hi Salil

You need to call deviceDao.refresh(yourDevice) in order to populate your foreign collections.

Cheers

Moritz

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[Salil Wadnerkar]

Hi Moritz,

My understanding is that refresh is necessary only if I modify the collection - via some other means than deviceDao.

If I am querying device from database, is the refresh necessary on all foreign-collections?

Anyway, I will try out your suggestion.

Thanks

Salil

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[Moritz Pfeiffer]

Hi Salil

As far as I know it is necessary to refresh foreign collections after a query.

You could also mark your foreign collection as foreignAutoRefresh=true, but I've never done that so I don't know exactly how that behaves.

Greetings

Moritz

[Salil Wadnerkar]

Hi Moritz,

The refresh does not help. Here is my modified code:

        List<Device> devices = (List<Device>)deviceDao.queryForEq("mac_addr", deviceMacAddr);

        if (!devices.isEmpty()) {

            Device device = devices.get(0);

            deviceDao.refresh(device); // <- REFRESH here

            ForeignCollection<Reading> readings = device.getReadings();

            CloseableIterator<Reading> iterator = readings.closeableIterator();

            List<Reading> result = new ArrayList<Reading>();

            while (iterator.hasNext()) {

                Reading reading = iterator.next();

                result.add(reading);

            }

            iterator.close();

            return result;

It still returns an empty collection.

regards

Salil

[Moritz Pfeiffer]

Hi Salil

The problem might be with how you setup/populate your foreign collection in the first place.
Does your Reading class have a database field called device with foreign=true?
Then when you instantiate your Reading initially do you associate it with the device properly like this?
Reading theReading = new Reading();
… set some values …
theReading.device = … the device created with deviceDao.create(theDevice) …
readingDao.create(theReading)

I believe the relation to the device has to be set before you call readingDao.create(theReading), but don't quote me on that.


BTW you can just create an ArrayList from the foreign collection like this:
ArrayList<Reading> result = new ArrayList<Reading>(device.getReadings());
As ForeignCollection is simply a Collection.

Greetings
Moritz

[Salil Wadnerkar]

Hi Moritz,

Yes, my Reading class has a device field, defined like this:

@DatabaseField(canBeNull = false, foreign = true, columnName = "device_mac_addr", foreignColumnName = "mac_addr")

private Device device;

I am not instantiating any readings directly. I need to get the readings from the 'correct' device specified by the Mac address. So, I am relying on the ForeignCollection field of the device to get the appropriate device readings.

And, thanks for the tip for generating ArrayList from Collection. :)

regards

Salil

[Moritz Pfeiffer]

Hi Salil

Readings aren't instantiated for you automatically.
Somewhere in your code you'll have to create them and associate them with the appropriate device so that the device's foreign collection gets populated properly.

Greetings
Moritz


Salil Wadnerkar <rohs...@gmail.com> schrieb:

[Salil Wadnerkar]

Hi Moritz,

I meant to say that devices and readings are already there in the database. I am merely querying the device (which is successful) and the corresponding readings through the ForeignCollection field of that device (which is not successful).

regards

Salil

[Moritz Pfeiffer]

Hi Salil

I understand the problem as being your inability to access the device's readings.
Perhaps the relations in your db aren't properly configured?
How did the Readings get into the db?
And when the readings were inserted into the db was the relationship between the reading and the device properly established?
I suspect that the readings are missing the foreign key of the associated device.

[Moritz Pfeiffer]

Hi Salil

The problem might be with how you setup/populate your foreign collection in the first place.

Does your Reading class have a database field called device with foreign=true?

Then when you instantiate your Reading initially do you associate it with the device properly like this?

Reading theReading = new Reading();

… set some values …

theReading.device = … the device created with deviceDao.create(theDevice) …

readingDao.create(theReading)

I believe the relation to the device has to be set before you call readingDao.create(theReading), but don't quote me on that.

BTW you can just create an ArrayList from the foreign collection like this:

ArrayList<Reading> result = new ArrayList<Reading>(device.getReadings());

As ForeignCollection is simply a Collection.

Greetings

Moritz

[Tom Maxwell-Mans]

Hey I'm having the exact same problem, so I'm wondering if this problem got solved?

 If it did I would really be interested in the answer 

thank you.

[Peter Lastameder]

I think I have the same issue... Did you solve this problem?