Adding Not Null constraint on field using PyQGIS

Question

I have a vector layer to which I want to apply some constraints using a script with PyQGIS (a NOT NULL constraint on some fields).

I checked the docs of PyQGIS and found setConstraintExpression (int index, const QString &expression, const QString &description=QString()) which as I understand it from the docs it applies to a QgsVectorLayer I used it in my code like this:

my_layer = QgsVectorLayer('./path/to/my_layer.shp', 'my_layer', 'ogr')
field_index = my_layer.fieldNameIndex('ID')
my_layer.setConstraintExpression(field_index, 'NOT NULL')

I get an error that the function does not exist :

> AttributeError: 'QgsVectorLayer' object has no attribute
> 'setConstraintExpression'

I also tried setFieldConstraint and I get the same error. How can I add a constraint on a field using PyQGIS? (QGIS 2.18 Las Palmas)

---

I've just noticed that these 2 methods I tried are in the QGIS API 2.99.0-Master which is still not released.

Answer 1

In QGIS v2.18.x you can use the QgsEditFormConfig class.

Set a Not Null constraint:

formConfig = layer.editFormConfig()
formConfig.setNotNull( fieldIndex, True )

Set a constraint expression:

formConfig.setExpression( fieldIndex, myExpression )

---

Not Null constraint in edit form:

Answer 2

Since the setters for field constraints won't be available until QGIS 3 on 29th September 2017, I couldn't wait that much to get this problem solved, so I worked around it by catching the event of featureAdded from the active layer in the qgis interface:

First I define a function to check the constraints :

def check_constraints(self, fid):
    feature_iterator = self.iface.activeLayer().getFeatures(QgsFeatureRequest(fid))
    feature = next(feature_iterator)
    code = feature.attributes()[0]
    if not code:
        self.iface.messageBar().pushMessage("Error", "the code of the entity is null !t", level=QgsMessageBar.CRITICAL)

and then I bind it the active layer of the QgsInterface

 self.iface.activeLayer().featureAdded.connect(self.check_constraints)

PS: I'm using it inside a class of a plugin that's why I use self.iface and not directly qgis.utils.iface

Answer 3

# Get the layer by its name
topo_details_layer_name = 'Topo_Details'
details_layer = QgsProject.instance().mapLayersByName(
            topo_details_layer_name)[0]
name_field_index = details_layer.fields().indexFromName('Name')
details_layer.setFieldConstraint(
            name_field_index, QgsFieldConstraints.ConstraintNotNull)