QGIS create selection script

Question

I am trying to create my first script in QGIS.

##Double attribute selection=name
##Vector=group
##Selection_layer=vector
##First_field=field Selection_layer
##expression=string 
##Second_field=field Selection_layer

from qgis.core import * 
from PyQt4.QtCore import *


layer = Selection_layer 
exp = QgsExpression("expression") 
query_1 = "First_field" + expression 
selection = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query_1)) 
selected_ids = [k[Second_field] for k in selection]

ids = [] for feat in layer.getFeatures():
    if feat[Second_field] in selected_ids:
        ids.append(feat.id()) 
layer.setSelectedFeatures([l for l in ids])

Now i got error like that:

And I would like to have my inputs to look like that:

The script is related to my previous question: Selecting new records using values inside previous selected rows in QGIS?. Now I would like to change it to run in as a script. What I am trying to do is to select 2 fields from current layer. For the first field, I need to create a selection query. For the second one, I need to select only records which have the same number as the records from first field.

Can you help me?

Answer 1

You get that error because you were trying to iterate over a name (i.e. Selection_layer) instead of a vector object. So you need to get it in this way for further using:

layer = processing.getObject(Selection_layer)

In your query_1 parameter, "First_field" doesn't convert the field to a parameter because, in this way, you have just defined a field called "First_field" (which doesn't exist). You may build the query in this way:

query_1 = '"'+ First_field +'" ' + expression

Finally, there were some other indentation errors, but I think they appeared when copying & pasting the code.

However, your final code should look like this (it worked for me):

##Double attribute selection=name
##Vector=group
##Selection_layer=vector
##First_field=field Selection_layer
##expression=string is NULL
##Second_field=field Selection_layer

from qgis.core import * 
from PyQt4.QtCore import *

layer = processing.getObject(Selection_layer) 

exp = QgsExpression(expression) 
query_1 = '"'+ First_field +'" '+ expression
selection = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query_1)) 
selected_ids = [k[Second_field] for k in selection]

ids = []
for feat in layer.getFeatures():
    if feat[Second_field] in selected_ids:
        ids.append(feat.id())
layer.setSelectedFeatures([l for l in ids])