Where to get QGIS 3 Join by Field Value Script?

Question

I´m trying to solve my problem with Join attributes by field value and for that I want to make a script on PyQGis to solve that.

Is there a location where I can found the script used in Join by field value on QGIS 3? To get it as a base and a guide.

I looked at GitHub and found only the Spatial Join script.

Answer 1

Since the QGIS version 3.6, you can export processing models as Python.

Just create a model with two input layers (geometry non required) and the "Join attributes by field" algorithm, save the model and export it as Python.

Result here :

from qgis.core import QgsProcessing
from qgis.core import QgsProcessingAlgorithm
from qgis.core import QgsProcessingMultiStepFeedback
from qgis.core import QgsProcessingParameterVectorLayer
from qgis.core import QgsProcessingParameterFeatureSink
import processing


class MyModel(QgsProcessingAlgorithm):

    def initAlgorithm(self, config=None):
        self.addParameter(QgsProcessingParameterVectorLayer('inputlayer1', 'input_layer_1', types=[QgsProcessing.TypeVector], defaultValue=None))
        self.addParameter(QgsProcessingParameterVectorLayer('inputlayer2', 'input_layer_2', types=[QgsProcessing.TypeVector], defaultValue=None))
        self.addParameter(QgsProcessingParameterFeatureSink('Output_layer', 'output_layer', optional=True, type=QgsProcessing.TypeVectorAnyGeometry, createByDefault=True, defaultValue=None))

    def processAlgorithm(self, parameters, context, model_feedback):
        # Use a multi-step feedback, so that individual child algorithm progress reports are adjusted for the
        # overall progress through the model
        feedback = QgsProcessingMultiStepFeedback(1, model_feedback)
        results = {}
        outputs = {}

        # join_attributes_by_field_value
        alg_params = {
            'DISCARD_NONMATCHING': False,
            'FIELD': 'join_field_layer_1',
            'FIELDS_TO_COPY': None,
            'FIELD_2': 'join_field_layer_2',
            'INPUT': parameters['inputlayer1'],
            'INPUT_2': parameters['inputlayer2'],
            'METHOD': 1,
            'PREFIX': '',
            'OUTPUT': parameters['Output_layer']
        }
        outputs['Join_attributes_by_field_value'] = processing.run('native:joinattributestable', alg_params, context=context, feedback=feedback, is_child_algorithm=True)
        results['Output_layer'] = outputs['Join_attributes_by_field_value']['OUTPUT']
        return results

    def name(self):
        return 'my_model'

    def displayName(self):
        return 'my_model'

    def group(self):
        return ''

    def groupId(self):
        return ''

    def createInstance(self):
        return MyModel()

Answer 2

If you have only one or two field you want to get I think the best way is to use field calculator to do it in one step.

I use it mainly after i have done a processing on a "source" layer (spatial join or whatever), and I want to get the information back on my "source" layer (and discard the temporary data after).

attributes(get_feature( '**layer_id_target**' , '**joining_field_target**', "**joining_field_source**")) [**field_name_you_want_target**]

For example after a spatial join :

attributes(get_feature( 'Couche issue de la jointure spatiale' , 'fid', "fid")) [name]