How to connect input parameter to the field calculator algorithm within the graphical modeler?

Instead of using the Field Calculator tool, a possible workaround is to create a custom script which allows the user to choose an attribute field and update it with a value they enter.

To do this, go to Processing Toolbox > Scripts > Tools > Create new script and copy the following:

##Update field by number=name
##Layer=vector
##Fields=Field Layer
##Number=number 0

from qgis.core import QgsExpression

layer = processing.getObject(Layer)  
layer.startEditing()

idx = layer.fieldNameIndex(Fields)
n = str(Number)
i = str(Fields)
e = QgsExpression(n + '*' + i)
e.prepare(layer.pendingFields())

for f in layer.getFeatures():
    f[idx] = e.evaluate(f)
    layer.updateFeature(f)

layer.commitChanges()

Save the script into C:/Users/You/.qgis2/processing/scripts. You will need 3 parameters:

• Layer - Vector layer
• Field - Table field
• Number - Number

Then add the script from the list of algorithms to your model. Use the Parent algorithms option if you want this script to run after a specific tool (i.e. at the end of the model).:

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Did a simple test where I want to update the id field by entering 5 in the Number parameter. Here is the attribute table initially:

Setting and running the model:

Result: