Skip child position of exported data based on field names

When exporting data of a xxx2Many field, the column to skip should be based on
the child field names instead of the exported value (which can be 0 for
example).

Closes #12316
(grafted from a91e1a9f95ca2b22d4ce5372b1753999d03aa8af)

--HG--
branch : 6.0

trytond/trytond/model/modelstorage.py

@@ -668,10 +668,11 @@ class ModelStorage(Model):
                         child_lines = ModelStorage.__export_row(child_record,
                                 child_fields_names)
                         if first:
-                            for child_fpos in range(len(fields_names)):
-                                if child_lines and child_lines[0][child_fpos]:
-                                    data[child_fpos] = \
-                                        child_lines[0][child_fpos]
+                            if child_lines:
+                                for child_fpos in range(len(fields_names)):
+                                    if child_fields_names[child_fpos]:
+                                        data[child_fpos] = (
+                                            child_lines[0][child_fpos])
                             lines += child_lines[1:]
                             first = False
                         else:

trytond/trytond/tests/export_data.py

@@ -9,6 +9,7 @@ class ExportDataTarget(ModelSQL):
     "Export Data Target"
     __name__ = 'test.export_data.target'
     name = fields.Char('Name')
+    value = fields.Float("Value")
 
 
 class ExportData(ModelSQL):

trytond/trytond/tests/test_exportdata.py

@@ -353,6 +353,26 @@ class ExportDataTestCase(unittest.TestCase):
                     'one2many/name']),
             [[export1.id, 'Target 1'], ['', 'Target 2'], [export2.id, '']])
 
+    @with_transaction()
+    def test_one2many_empty_value(self):
+        "Test export_data one2many with first child with 0 value"
+        pool = Pool()
+        ExportData = pool.get('test.export_data')
+        ExportDataTarget = pool.get('test.export_data.target')
+
+        export, = ExportData.create([{}])
+        ExportDataTarget.create([{
+                    'value': 0,
+                    'one2many': export.id,
+                    }, {
+                    'value': 42,
+                    'one2many': export.id,
+                    }])
+
+        self.assertEqual(
+            ExportData.export_data([export], ['id', 'one2many/value']),
+            [[export.id, 0], ['', 42]])
+
     @with_transaction()
     def test_reference(self):
         'Test export_data reference'