Joining objects with bpy.ops.object.join doesn't respect selection state

Question

Where is there an issue with my code? I want to merge the models under the empty object into one object and save it under each empty object, but after I run the code, it merges all the models into one model

import bpy
import time
class MergeTestModelsOperator(bpy.types.Operator):
    bl_idname = "object.merge_test_models_operator"
    bl_label = "Merge Test Models"
    bl_options = {'REGISTER', 'UNDO'}
def mergemodel(self, mergeobj):

    # Merge other models into new objects
    for obj in mergeobj.children:
        if obj.type == 'MESH' and obj:
            obj.select_set(True)
            bpy.ops.object.join()
            bpy.context.view_layer.objects.active.name = mergeobj.name.replace("_MODEL", "")
            bpy.ops.object.transform_apply(location=True, rotation=True, scale=True)

def execute(self, context):
    # Obtain all models in the scene
    objects = bpy.data.objects

    start = time.perf_counter()

    for obj in objects:
        if obj.type == 'EMPTY' and obj.name.find("_MODEL")!=-1:
            self.mergemodel(obj)
            # self.report({'INFO'}, f"EMPTY: {obj.name}.")
            continue

    end = time.perf_counter()
    runTime = end - start
    runTime_ms = runTime * 1000

    print(f"Time: {runTime}\n Time_ms: {runTime_ms}")
    self.report({'INFO'}, f"Time: {runTime}\n Time_ms: {runTime_ms}")

    # Display successful prompt information
    self.report({'INFO'}, "Test models merged and deleted successfully.")

    return{'FINISHED'}


class MergeTestModelsPanel(bpy.types.Panel):
    bl_label = "Merge Test Models Panel"
    bl_idname = "PT_MergeTestModelsPanel"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = 'Tools'
    bl_context = "objectmode"
def draw(self, context):
    layout = self.layout

    # Add a button that will call the Merge Test Models Operator operator when clicked
    layout.operator("object.merge_test_models_operator")


def register():
    bpy.utils.register_class(MergeTestModelsOperator)
    bpy.utils.register_class(MergeTestModelsPanel)
def unregister():
    bpy.utils.unregister_class(MergeTestModelsOperator)
    bpy.utils.unregister_class(MergeTestModelsPanel)
if name == "main":
    register()

Answer 1

I think it's caused by selection state not being reset after each iteration. You select objects but don't unselect the previous ones. So the last empty children get joined with the previous ones.

You can either play with the selection state or use a context override which is IMO more suited and will work both for object.join and object.transform_apply.

import bpy
def mergemodel(self, mergeobj, context):
    objects_to_merge = [o for o in mergeobj.children if o.type == "MESH"]
    if len(objects_to_merge) < 2:
        return
    with context.temp_override(active_object=objects_to_merge[0], selected_editable_objects=objects_to_merge):
        bpy.ops.object.join()
        objects_to_merge[0].name = mergeobj.name.replace("_MODEL", "")
        bpy.ops.object.transform_apply(location=True, rotation=True, scale=True)
def execute(self, context):
    objects = bpy.context.scene.objects

    for obj in objects:
        if obj.type == 'EMPTY' and "_MODEL" in obj.name:
            self.mergemodel(obj, context)