fix reading Mergin project settings saved from QGIS 4.0 to 3.x

Co-authored-by: Copilot <copilot@github.com>

Author: JanCaha

Mergin/project_settings_widget.py

@@ -20,6 +20,7 @@
     QgsMapLayer,
     QgsVectorLayer,
     QgsFieldProxyModel,
+    Qgis,
 )
 from qgis.gui import (
     QgsOptionsWidgetFactory,
@@ -51,6 +52,7 @@
     excluded_layers_list,
     get_fields_for_checkbox,
 )
+from .qgis_properties_version_4 import read_mergin_properties, is_qgis_version_4
 
 ui_file = os.path.join(os.path.dirname(os.path.realpath(__file__)), "ui", "ui_project_config.ui")
 ProjectConfigUiWidget, _ = uic.loadUiType(ui_file)
@@ -82,6 +84,11 @@ def __init__(self, parent=None):
         QgsOptionsPageWidget.__init__(self, parent)
         self.setupUi(self)
 
+        if Qgis.versionInt() < 40000 and is_qgis_version_4(QgsProject.instance().fileName()):
+            props = read_mergin_properties(QgsProject.instance().fileName())
+            for key, value in props.items():
+                QgsProject.instance().writeEntry("Mergin", key, value)
+
         self.cmb_photo_quality.addItem("Original", 0)
         self.cmb_photo_quality.addItem("High (approx. 2-4 Mb)", 1)
         self.cmb_photo_quality.addItem("Medium (approx. 1-2 Mb)", 2)

Mergin/qgis_properties_version_4.py

@@ -0,0 +1,67 @@
+import xml.etree.ElementTree as ET
+from typing import Dict
+import zipfile
+
+
+def read_xml_tree_from_qgz(qgz_path: str) -> ET.Element:
+    qgs_filename = next(
+        (name for name in zipfile.ZipFile(qgz_path).namelist() if name.endswith(".qgs")),
+        None,
+    )
+
+    if qgs_filename is None:
+        raise ValueError(f"No .qgs file found inside {qgz_path}")
+
+    with zipfile.ZipFile(qgz_path, "r") as input_zip_file:
+        entries = {name: input_zip_file.read(name) for name in input_zip_file.namelist()}
+
+    return ET.fromstring(entries[qgs_filename])
+
+
+def is_qgis_version_4(qgz_file: str) -> bool:
+    root = read_xml_tree_from_qgz(qgz_file)
+
+    version = root.attrib.get("version", "")
+    if not version.startswith("4."):
+        return False
+
+    return True
+
+
+def parse_properties(element, prefix="") -> Dict:
+    """Recursively parse nested <properties> elements into a flat dict with path keys."""
+    result = {}
+
+    for child in element:
+        if child.tag != "properties":
+            continue
+
+        name = child.attrib.get("name", "")
+        key = f"{prefix}/{name}" if prefix else name
+        prop_type = child.attrib.get("type")
+
+        if prop_type is not None:
+            if prop_type == "QStringList":
+                result[key] = [v.text for v in child.findall("value")]
+            else:
+                result[key] = child.text
+        else:
+            result.update(parse_properties(child, prefix=key))
+
+    return result
+
+
+def read_mergin_properties(qgz_file: str) -> Dict:
+    root = read_xml_tree_from_qgz(qgz_file)
+
+    version = root.attrib.get("version", "")
+    if not version.startswith("4."):
+        return {}
+
+    mergin_elem = root.find(".//properties[@name='Mergin']")
+    if mergin_elem is None:
+        return {}
+
+    props = parse_properties(mergin_elem)
+
+    return props