feat: only offline the subset that matches all layer's filters

If a project has layer1 and layer2 that have the same data source,
but different applied filters, download only the features that match
either layer1 or layer2 filter and not all the features from the
datasource.

Author: suricactus

libqfieldsync/offliners.py

@@ -403,12 +403,13 @@ def _convert_to_offline_project(
         driver = ogr.GetDriverByName("GPKG")
         data_source = driver.CreateDataSource(offline_gpkg_path)
         datasource_mapping = self._get_datasource_mapping(project, offline_layers)
+        filters_mapping = self._get_filters_mapping(datasource_mapping)
 
         for layer_infos in datasource_mapping.values():
             layer_to_offline = layer_infos[0].layer
             self.create_layer(layer_to_offline, data_source, offline_gpkg_path)
 
-        for layer_infos in datasource_mapping.values():
+        for datasource_hash, layer_infos in datasource_mapping.items():
             request = QgsFeatureRequest()
             # All layers for given `datasource_hash` are pointing to the very same file/datasource.
             # Here we get the first layer for convenience, but it doesn't really matter.
@@ -439,6 +440,9 @@ def _convert_to_offline_project(
                 layer_bbox = tr.transform(bbox)
                 request.setFilterRect(layer_bbox)
 
+            if filters_mapping[datasource_hash]:
+                request.setFilterExpression(filters_mapping[datasource_hash])
+
             source = self.convert_to_offline_layer(
                 layer_to_offline, data_source, offline_gpkg_path, request
             )
@@ -465,6 +469,26 @@ def _convert_to_offline_project(
             project.writePath(offline_gpkg_path),
         )
 
+    def _get_filters_mapping(
+        self, datasource_mapping: Dict[str, List[LayerInfo]]
+    ) -> Dict[str, str]:
+        """Get mapping of filter strings to be applied for each datasource. If no filters to be applied, the value will be an empty string."""
+        filters_by_datasource = {}
+
+        for datasource_hash, layer_infos in datasource_mapping.items():
+            filters = []
+            for layer_info in layer_infos:
+                if layer_info.subset_string.strip():
+                    filters.append(f"({layer_info.subset_string})")
+                else:
+                    # if there is no subset string, we don't need to apply any filter, as it means "all features"
+                    filters = []
+                    break
+
+            filters_by_datasource[datasource_hash] = " OR ".join(filters)
+
+        return filters_by_datasource
+
     def _get_datasource_mapping(
         self,
         project: QgsProject,