QGIS batch merging grouped layers

Question

I have a situation as you see below:

I have imported the .dxf file from AutoCAD, which finally looks like the above. Each layer includes lines and polylines. I need to have them merged with each other.

I found some similar problems here:

Batch process for merging features in multiple layers

Merging multiple temporal inputs in QGIS Graphical Modeler

which utilizes the graphical modeler. It does make sense, although I haven't used the graphical modeler so far, therefore I don't know how to start.

Is there a chance to make this merge in one go?

Answer 1

If you are open to a PyQGIS solution, it can be done very simply:

p = QgsProject.instance()

# get all layers in project
lyrs = p.mapLayers().values()

# filter layers if necessary (could also filter by geometry type)
line_layers = [x for x in lyrs if 'line' in x.name().lower()]

# merge layers (don't forget to specify your coordinate system)
merged = processing.run("native:mergevectorlayers", {'LAYERS':line_layers,'CRS':QgsCoordinateReferenceSystem('EPSG:4326'),'OUTPUT':'TEMPORARY_OUTPUT'})['OUTPUT']

# add to canvas
p.addMapLayer(merged)

To merge layers per group (using the hierarchy in your screenshot):

# reference to the project
p = QgsProject.instance()

# reference to the layer tree
root = p.layerTreeRoot()

# reference to the group called 'Batch'
group = root.findGroup('Batch')

# loop through all the items in the Batch group
for child in group.children():
    # find sub-groups
    if isinstance(child, QgsLayerTreeGroup):      
        sub_group = child
        
        # empty list to store the layers ready to merge
        lyrs_to_merge = []

        # find line layers in the sub-group
        for sub_child in sub_group.children():
            # check whether the child is a layer and has 'line' in its name
            if isinstance(sub_child, QgsLayerTreeLayer) and 'line' in sub_child.name().lower():
                lyr = sub_child
                # add to list
                lyrs_to_merge.append(lyr.layerId())
        
        # perform merge
        merged = processing.run("native:mergevectorlayers", {'LAYERS':lyrs_to_merge,'CRS':QgsCoordinateReferenceSystem('EPSG:4326'),'OUTPUT':'TEMPORARY_OUTPUT'})['OUTPUT']    
        
        # name the merged layer
        merged.setName('merged | ' + sub_group.name())
        
        # add layer to project, but prevent adding to canvas and layer tree (with False) 
        p.addMapLayer(merged, False)
        
        # insert the merged layer at the top of each sub-group
        sub_group.insertLayer(0, merged)

Result: