How do I set an attribute value to NULL for a feature?

OrestHalustchak — Wed, 13 Mar 2019 18:49:29 GMT

I'm having an issue when I try to set an attribute value to null in the process of adding a new feature to a feature table.

I'm using C++ Qt Runtime sdk 100.3.

I have a feature layer that is enabled for editing. The feature table has string and int attributes that are defined as Editable = yes and Allows Null Values = yes.

What I'm doing is this:

Create a new feature using ServiceFeatureTable::createFeature().
Create a QMap<QString, QVariant> of feature attributes.
Create a null QVariant value: QVariant qv = QVariant();
I verified that qv.IsNull() is true;
Add to the attributes list: featureAttributes.insert("MyAttrName", qv);
I used this for both string and int attributes.
Create a new feature: featureTable->createFeature(featureAttributes, geom, this);
If I go ahead and add that feature to the table and apply edits, then I see that the resulting attribute is not null but has a non-null empty string for string attributes and 0 for int attributes.
What I also found was that if right after creating the feature, I get the attributes back via the AttributeListModel, the attributes are no longer null.
AttributeListModel* alm = feature->attributes();
QVariant qv2 = alm->attributeValue(QString("MyAttrName"));
qDebug() << "qv2 is null =" << qv2.isNull();
This returns false. I expected it to return true.

If I leave out the attributes from the attribute list altogether, then I do get null values in the resulting feature in the table. However, that approach will not work if the attributes include default values that will set them to non-null values and that approach will not work for ServiceFeatureTable::updateFeature();

I need a way to set values to null explicitly.

Does anyone have suggestions as to what to try? I am probably missed something.

Regards.

Re: How do I set an attribute value to NULL for a feature?

JamesBallard1 — Thu, 14 Mar 2019 22:50:02 GMT

Orest Halustchak‌, yes this can be done. We make a distinction between invalid QVariant, for example if someone requested the value for a field that doesn't exist and legal, valid "null" values for fields whose value is actually "null".

const QVariant jsonNull(QJsonValue()); // this is a "null" value.

It's explained here in more detail: AttributeListModel Class | ArcGIS for Developers

Please let us know if that works for you.

Re: How do I set an attribute value to NULL for a feature?

OrestHalustchak — Tue, 19 Mar 2019 13:58:43 GMT

That was it. It worked.

Thanks!

Orest.

topic Re: How do I set an attribute value to NULL for a feature? in Qt Maps SDK Questions

How do I set an attribute value to NULL for a feature?

Re: How do I set an attribute value to NULL for a feature?

Re: How do I set an attribute value to NULL for a feature?